To install the Support Toolkit, upload the complete "stk" directory into your forum's root directory on your server (the one containing a file named "config.php"). Once you have uploaded the directory into the correct place, point your browser towards the toolkit to access it (http://www.example.com/phpBB/stk/). In the "contrib" folder you will find some additional installation instructions (in the MODX format) which will allow you to better integrate the STK with your phpBB installation.
To update the Support Toolkit you only have to overwrite the old files with the files of the latest release.
Note: When updating from any RC release make sure that you first remove all Support Toolkit files from your server (empty the "stk/" folder) than upload all the files that are included in the new package to this location.