How do I keep old versions of files in Delta Drive?

Turn on Keep old versions for a folder (right-click → versions, or ⌘K). Then replacing a file leaves the previous copy behind so you can go back to it.

There are two settings: Keep old versions (on or off) and How long they're kept — Forever, a number of months, or until a date. Both are inherited from the folder above, and the screen says which folder a setting comes from. Turning versions off stops new ones being kept; it never removes what's already there.

See why some lengths aren't offered and what versions cost.