What version of Premiere Elements and on what computer operating system?
Let us assume Premiere Elements 11, 12, or 13 on Windows 7, 8, or 8.1 64 bit for the purpose of an example.
1. Select all the clip on the Timeline (all at one time) that are being targeted for same duration change.
2. Right click anywhere in the selection, select Time Stretch, and, then in the Time Stretch dialog, change the duration 00;00;0O;00. (hours; minutes; seconds; frames).
Do you have one of those versions that I mentioned, making sure to select all the involved clips ahead of time before going
after Time Stretch?
If you want all the clips on the Timeline, then Edit Menu/Select All
Hold down the Shift key of the computer main keyboard, and, with the mouse cursor, click on each of the clips to be involved in this duration change
With the mouse cursor, draw a rectangle around the clips to go into the selection in order to select them
Any questions or need clarification, please do not hesitate to ask.