I cannot find instructions on upgrading.  They really should be somewhere obvious on the documentation page and the forum search really leaves something to be desired.
Anyway, I have version 2.3.  I have created a few extra fields and groups, but not a lot.  How do I upgrade? Do I just upload the new version and it overwrites everything?  Are my custom extra groups/fields in danger?  Where is the guide for upgrading?

Thanks in advance.

If you take a look at your phpmyadmin you will find that k2 has a table jos_k2_extrafieldsgroups and jos-K2_extrafields. This is where the actual definiition of the xtra fields are stored.
Please backup your site before upgrading. I recommend akeeba backup Pro which is paid component.
Most of the time upgrading K2 will effectively "toast" any changes you have made to the core of the component (as it will if you upgrade any other Joomla component that you have modified), so if youve hacked it, upgrading can be painfull. Making all your changes as overrides is the best way. See Here. Plugins are another way to make changes that will withstand upgrades, see Here.
There is a one line upgrade instruction in this blog post .
All of the above comes from extensive reading, and searching over time.
As you next discussion asks, are there paid solutions. Yes, Many extensions exist for k2 on the joomla extension directory, and there are many dev's that will help you with k2. IMHO, If i get a "paid" extension, it comes with some sort of support, but usually doenst do exactly what i want and has to be changed, K2 and others are free, and you just get right to the changing part. See friend request and pm me if you need more info
wjwjr

Awesome thanks for the help William. It will probably be tomorrow before I get back to working on this particular clients site that is running K2 but I'll definitely let you know if I need further info.