I would like to back up files (different types). I would like to back up the difference (delta) between the previous version (last saved to current). In addition the process has to be done in parallel in different locations. Is there a gratis that I can use to do it automatically?
Veeam Endpoint Backup is free, and allows you to create incremental backups - and initial seed backup and multiple, incremental backups. In my limited experience (working with large file stores - 500GB+) it was slow working with the file system, but quicker when backing up the full volume.
Another popular option is Duplicati. If anything, this is what I would look into next myself, but not having directly used it myself, my recommendation is based on prior reading and hearsay. Again, Duplicati can handle incremental backups. Duplicati v2 is still considered 'experimental' - you'll have to make your on decision on which to opt for.