If you want to add or update content to a set that is previously in the system you can do the following:

1) Add the content to the set you wish to update on the server and make sure permissions are there.
2) Edit the set in the CMS and click the save button. This should cause the system to re-index the files and folders, and inventory the change.
3) If you have added new photos or removed photos you'll need to recalculate the content totals by going to the Batch Operations Tool (Located under Utilities >> Batch Operations >> then select the content and move to the right-hand side then select "Calculate Image Sizes / Movie Information" option and click save.

When this is done the new content should be added and updates.