Welcome to XBMC.
First off, can you continue this by making a thread in the XBMC for Xbox Specific Support
section, you will get more help there. I would rater keep this thread clean of unrelated topics.
Anyways, you need to find the old XBMC installation directory on the xbox and replace. Common places are:
I would reccommend connecting to xbox via ftp from a different dashboard (EvoX, UnleashX, etc), then deleting the XBMC folder and replace it with the updated XBMC. If you want, you should backup the userdata folder to save all your old settings and library. So for example, if "E:\XBMC\" was on your xbox, you would just replace it with the "XBMC" folder from the T3CH build or sshcs.
After the update, you can then install the Justin.tv plugin from the SVN Repo Installer (comes pre-installed on the t3ch builds) or by manually downloading the plugin and placing it in ...XBMC\plugins\video.