Are you just trying to change which theme is the Active one?
You cannot change which theme is the "Activated" one by using FTP, at least not that I know of. The Active Theme is set in the Database. If you are unable to log into the Dashboard to change it there, I would suggest you edit the database to change the Active theme.
Bluehost has a tool in their cPanel called phpMyAdmin for doing that. You will want to select the correct database name from the left, then choose the table called wp_options or something similar to that. On the 2nd or 3rd page of the 'options' table there will be 2 lines for 'template' and 'stylesheet' and you have to edit both to type in twentyten (one word, lowercase).