Just download (from the release board at the top of the forum) and install MC11. When installing, you'll get an option to uninstall MC10, my advice would be to choose no for now, (MC10 & MC11 can co-exist on the same computer), you can always remove MC10 later.
If you haven't already had MC11 installed, you'll now have 30 days to evaluate it or you can purchase at any time by going to the help menu. When you purchase you should get the $26 discount upgrade price. If not, post back here and JRiver will have to help you out.
Once MC11 is installed you have 2 options to transfer your library data from MC10 to MC11:
1. Backup your MC10 Data folder to somewhere outside of your J River folder for safe keeping. Then in MC11 when restoring the library point MC11 at the backup folder in the MC10 folder. However, this makes the data unusable in MC10 should you ever want to go back to it, therefore I would recomend the following:
2. Copy your MC10 backup & jmr files to Documents & Settings>Administrator>(folder options: unhide hidden folders)Application Data>J River>Media Center 11>Library Backups. Now you can restore your library in MC11.