login into SMF via front end then select the ADMIN tab on the forum (usually at the top under the user info)
Then on left side you will see Themes and Layout (select this)
Scroll down to Install a New theme. browse then INstall..
then you will need to go back to Themes and Layout and in the dropdown select the theme you just installed.
If you are using the JoomlaHacks bridge you should go to joomla admin then open joomla_smf forum configuration and simply click the safe iscon. this will help clear the cache.