How do I delete previous projects

Every time I start a project I set my project and work files to F:\Adobe

-I work in EXPERT view so I may control all aspects of my project
(F: is MY drive for temporary files, your drive letter may be different)

Click EDIT and then PREFERENCES and then SCRATCHDISKS
I set every one of the options to F:\Adobe
PrE then automatically creates sub-folders as needed
F:\Adobe\Adobe Premiere Elements Audio Previews
F:\Adobe\Adobe Premiere Elements Captured Audio
F:\Adobe\Adobe Premiere Elements Captured Video
F:\Adobe\Adobe Premiere Elements Video Previews
F:\Adobe\Encoded Files
F:\Adobe\Layouts
F:\Adobe\Peak Files

Both Premiere Elements and Premiere Pro work the same.   You may have to look into each project to see where the scratch disk files were stored.  

For the future, consider creating  a dedicated folder for each new project.  Put your project there and set your scratch disks to record there.   If you like the security of working with media copies, put them there too.  When you're done you can move or delete the entire folder.  

You use Windows File Explorer the same as you would do for any other file that you want to delete

Go to the Search bar at the top of http://www.microsoft.com/en-us/ to find out how to use File Explorer

I have Adobe Premiere Pro 2020