I guess you meant 3.4.6, which was the last version which supported macOS 10.13.
You have the option to keep your synced database in the format used by 3.4.6. The only new feature which won’t be available is the floating time zones (introduced with 3.6 and requiring a database migration). If you’ve already accepted the migration prompts however, you’ll have to restore it from an older backup.
Hi, restoring a backup that was taken prior to migrating to the latest database format is one option. However, if this happened a while ago, that may not be very ideal. In that case, we can help to downgrade your current database to the previous format, which is sync-compatible with v3.4.6. If you would like our help with this, please just send us an email.