Themes Purchased from Tumblr
If you purchased a theme directly from Tumblr, it should automatically be installed on the blog for which it was purchased. If you encounter an issue with this not having worked correctly, please file a support ticket and we'll help out however we can.
Themes Purchased Elsewhere
If you've purchased one of our themes elsewhere just follow the instructions that are included in the "install.html" file included with the theme. Double-click it to open it, then everything should be clear!
It might take a few minutes for the theme to update on your site, and we would recommend keeping a copy of the code that you overwrite.