You can still use paperclip for attachment management by setting SPREE_USE_PAPERCLIP environment variable to true, but keep in mind that paperclip is DEPRECATED and we will remove paperclip support in Spree 4.0.
Install missing migrations
You're good to go!
Read the release notes
For information about changes contained within this release, please read the 3.6.0 Release Notes.
Verify that everything is OK
Run your test suite, click around in your application and make sure it's performing as normal. Fix any deprecation warnings you see.