I am a new user of restic.
I want to use restic to schedule creating backups and save the backup onto Dropbox. An incremental backup is ok for 6 times, but after 6 times restic should create a full backup.
The local copy of the backup which are on Dropbox can be deleted.
What is the best way to set up restic?
@guidovanharten Welcome to the community and the forum.
What is your definition of a “full backup”? The closest thing to what you are thinking is probably to ask restic to read through all files when backing them up, which is done with the -f or --force argument to the backup command.
Restic will however still deduplicate the data, so it will never send data that is already in the repository.
Put another way; You don’t need to think in terms of full and incremental backup with restic. Simply forget about that and your life will be easier 
To forget and prune data in your repository, you use the forget and prune commands.
@rawtaz, thanks for your reply.
Better studying of the docs I realize my fault.
Now I have to figure out how to backup the data of my Nextcloud on a regular basis which I can restore if needed.
Restic only supports backup of ‘local’ data. But you could mount your Nextcloud account (e.g. via rclone) and then backup the mount with restic. Note though that this is not very efficient and you might need to specify --ignore-inode for subsequent backups to speed things up.
If backing up nextcloud, it would probably be better to backup the data directory, configuration file, and database dump instead of just grabbing the files.