Branch lines (or main line) is not directly associated with a database.
If you want to have up to date information in your development instance (i.e. on your machine), you can create a database there, populate it from a database backup with the correct information and point your branch, or main, to that database.
If you want to have up to date information in the cloud, for instance Acceptance or Production, simply restore a database backup into the environment you are working with. After restoring the database, you can choose which development line to put into that environment (main or branch) and it will use the database you have just restored.
Does that help?
Well you restore a database to the environment. If you deploy your branch line to the environment where you have the database restored from mainline, it would work for the branch line as well. I assumed you want to do this on cloud.
If you want to do this locally, just restore your db to postgres: https://docs.mendix.com/developerportal/operate/restore-backup-locally
Hope this helps
The Built-In Database of Mendix Uses an H2 In-Memory Database. Your Backup is most probably a proper Postgresql or MySQL / Oracle database and the backup formats are not interchangeable. As proposed earlier, create a new Database matching your Original DBs Type, Import the backup and Reconfigure the App to Talk this DB instead of the Default Built-In one.
IF your data is lightweight enough and you don’t want to mess with Relational Databases you can use a Neutral Format, Like Export / Import of Excel and CSV Files