Show Versions
After every edit operation on rollup, it creates a new version of it, so even if you need the previous Rollup you can access it from there.
Edit
If the user wants to edit the existing action click on Edit. But a user can edit only the Label, and Description on the first page and add, and remove roll-up types, and custom filters on the next page. But the user cannot edit the previously defined object path.
Recalculate
If the user needs to calculate the newly created action on the existing records then select the Recalculate option and can apply the custom filter to identify which records the user wants to consider for recalculation operation.
Note: It may take time to recalculate existing records. So the user doesn’t need to stay on the same page, once the recalculation procedure is completed the application sends an email to the user who initiated the Recalculate process.