it's simple, you can do it 2 ways, go onto your storm's browser, navigate to the crackberry forums storm thread section, check out the theme you want, then click the OTA link that should be in the original post. then it'll do it for you 1,2,3.
or you can do it the way I do it, do the same thing but from your computer, right click the OTA link, click copy shortcut, then send an email to your phone with the link. when you get the email, just open it, click the link in there and it'll do it for you 1,2,3
