I’d recommend installing Rig Manager on your PC or Mac, and connect up to the Kemper. It’s possible to edit on the Kemper itself, but Rig Manager makes things so much easier.
I’d make a backup of the Kemper.
Then I’d “initialise globals”. As it’s second hand, the previous user may have altered all kinds of settings, in ways you might not expect. Initialise globals is in the System menu:
““Init Globals” resets most global parameters to their default values. This includes all System Settings. It also includes the settings in the Output Section, which is by definition global and not Rig-specific.”
You already mentioned a software update (if it’s not the latest OS).
Download the manual and begin reading through it.
Also check out the tutorial videos on YouTube.
And have fun 😀
Here are the essential resources.
Documentation
👉 Kemper Manuals and Quick Start guides 📖
👉 Rig Manager Download and Documentation 💽📖
👉 Kemper Tutorials & Demos (videos) 🎬
👉 Kemper FAQs (Frequently Asked Questions)
✔Download Main Manual 11.0 English
👉 Before you buy Commercial Rigs and Profiles check out the Rig Manager for Rig Packs and Rig Exchange
Essential Videos for Beginners and Veterans
Kemper Tone Tips - Volume 1 - Testing an Amp Profile
Kemper Tone Tips - Volume 2 - Profile Tweaking
Kemper Tone Tips - Volume 3 - Kemper Kabinet
👉 Yes, you can change a Profile - The Biggest Myth About the Kemper
I would add:
1) work out what you want it to do e.g. set up for live with a clean sound, crunch and solo sound with xxx effects that I can switch on and off. That was you don;t get overwhelmed with functionality
2) Use Rig manager to "audition rigs and sounds
3) If you find a good sound, save it and come back to it. Personally I prefer not to "overtweak"...if the sound isn;t right, move on. Don;t try to make a Fender twin profile sound like a Marshall JCM800
4) Check out Morph
5) For live use performance mode
6) If connecting to a mixing desk or interface, check that the output has the -12db box checked
All good suggestions.
+1 on performance mode for live use especially, but I find it very useful at home. Some kind of foot controller is almost mandatory for live use. I tried a couple different 3rd party controllers that I happened to already own. In the end, I concluded that the remote is the best solution. My recommendation is don't waste your time and get a remote ASAP.
Then there is the BIG question of speakers...
i recommend getting the main manual first.
a lot of things are very different from other gear, understand the concepts behind browser vs. performance mode, morphing etc.
no need to read everything, but you will miss a lot otherwise.
i would use it without Rig Manager first. i believe it´s less confusing to use it standalone,
until you know what you want from Rig Manger.
Don´t forget to plugin and play 
welcome!
Welcome to the Party!
I'm surprised no one mentioned the most important number one thing to do -- buy more guitars!
First of all ; last update...
Toaster is old and everyone doesn't put the last one ; you can miss : Delay, reverb 
(it's beginning to be an old update), Kone/imprints, Fuzz/drives, Liquid....
After that, try to understand how a profiler work and what means that special vocabulary (rig/imprint/liquid/Perfomance, etc....).
It will be more or less complicated following what you onwed before and how you used it (if you go in detail)
Display Morei recommend getting the main manual first.
a lot of things are very different from other gear, understand the concepts behind browser vs. performance mode, morphing etc.
no need to read everything, but you will miss a lot otherwise.
i would use it without Rig Manager first. i believe it´s less confusing to use it standalone,
until you know what you want from Rig Manger.
Don´t forget to plugin and play
welcome!
I think it depends on the person.
I have never read the manual but I have referred to it when I hit a problem.
The KPA is fairly intuative so its not essential, but if you can take the time to read it then its definitely sensible...
Its well written as well....very readable...
Welcome To The Family 
Hello and Welcome ✌️
Buy the Tone junky everything pack for 99$ 😆
In all seriousness, a factory reset is a great idea with a used United.
And then I would recommend to NOT yet dive into 100s or 1000s of profiles. Flip through a few until you find one that kinda does what you want an stick with it for a couple of days. Try effects, fiddle with the eq, dive into the stack settings. So that you get a feel for the overall function.
Only then I would start looking for different profiles. The reason is that with 10s of 1000s of profiles available both commercial and for free it can get pretty overwhelming st the start. You don't even know how to use it and are already tasked with finding a needle in a haystack that suits you best.
Focus on one and develop from there would be my advice
