Just now upgrading from 139h to 153.

Playing it safe, I took a back-up of the site files and uploaded them into a /store subfolder. Checked the store and admin, all fine.
Then I uploaded all the new v153 files into that folder, and corrected the configure files for the /store/ path.

White screens for store and admin.

I suspect I need to upgrade the database but I've forgotten how to do that.

Any pointers? Other ideas?