If you would like to use a non-equal (or "historical") temperament that is not already in your device, then you can add it with this screen.  You only have to enter the 12 offsets that define the temperament, and then give the new temperament a name and tap on the "Save" button, which appears after you give the temperament a name.  Then the temperament will be available for you to apply to any tuning.

If you would like to shift all the offsets of an existing temperament so that the average offset is zero, or so that the offset for the note A is zero, then you do not have to make a separate temperament. You can select one of the normalizing options available on the "Load a non-equal temperament" page.