With your Basic account, you can back up the last year* of one of your Timeline projects!!!
It's easy to change the project you're backing up.
On the iOS app
Tap on your Profile picture at the top left of the home screen > Settings (gear icon at the right) > Backup and Restore > Change > [project] > Done.
On the Android app
Simply go to the app's Settings > Backup & Restore > Change > [project] > Done.
On the Android app, you can also go to the Projects menu and swipe left on a project to select it for backup.
Keep in mind that you can only back up one project for one year when using a Basic account.
*the length of the Basic account backups was extended from 30 days to 365 days on Nov. 25, 2019. Any backup created before that only contains 30 days of snippets.