As a reseller, you can set the minimum amount for how many versions of a single file will be saved, and for how long these versions are kept before deletion.
To get to the History settings, click on the button, then on History in the top navbar. Use the slider buttons to edit the settings, then click on the newly-appeared Save button to apply the changes. If you want to cancel, just leave the page - changes will not be applied.
Please note that these are minimum amounts, despite the name suggesting that these are maximums.
These are reseller-level settings that are inherited by all sub resellers and companies under you. They can only increase these values, but not reduce them. This is done in order to ensure that each and every company has at least a certain level of file versioning. You can, of course, lower this amount to 0, but that means that no older versions of files will be retained by default.