If I need to find it, I tend to use the global search at the top of the navigation panel. Just type 'Periodic Project Report Template' and you should get the file location.
Once you've gone to the file, you can download it and make any changes you need. I've always saved a copy of the original on my hard drive too, just in case I need to start again!
When you're ready to upload the new template, that is done using the 'Actions' in the nav bar.