Hi Sandy,
Right click on any psd file, choose 'Open With' - choose another app
Check the box marked 'Always use this app to open psd files'
Scroll to the bottom and click 'More Apps'
If PS Express still does not appear in the list, scroll to the bottom and choose 'Look for another app on this PC'
Navigate to C:\Program Files\Adobe or C:\Program Files (x86)\Adobe
Find the PS Express executable and choose that
This should set the files association on the psd files for you