First, you will need to make sure that the session tempo matches the audio tempo. You did record to the session tempo and a click, didn’t you? If not, you will have to determine the session tempo by using Identify Beat or Beat Detective on the current audio, and map out the session tempo. If the sessions wasn’t recorded to a session tempo and click, and the tempo varies widely throughout the session, this task becomes exponentially more difficult, but I won’t get into that here, I’ll just assume the session in question has a session tempo, was recorded to a click, and is one tempo throughout.

Next, Consolidate all regions on the tracks to whole regions throughout the session. This saves steps later.

Next, you will need to put Pro Tools in Grid mode, set the grid to Bars|Beats and then trim the ends of all of the audio regions to a grid point, and set a marker there. This will make one of the next steps much easier.

Next, you will need to change the session tempo to the new tempo that you wish, ensuring first that all audio tracks are set to "Sample Based" and not "Tick Based".

Last, you will need to use the TCE tool to compress or expand the existing audio regions to the new tempo. Because you trimmed the regions to bar|beat markers and consolidated all reagions into whole regions, all you need to do is drag the ends of the regions with the TCE tool to the marker that you created.

The quality of the results depends largely on how drastic a tempo change you are attempting, and which TCE plugin you are using.