You 'Odin' back to stock. Google can help you more than I can, I've never done it, but this link will give you an idea of the process.
How to install stock official Samsung Galaxy S4 firmwares with ODIN | NaldoTech
Alternately, you could flash one of the ROMs that are popping up that are based on the new firmware. The new ones are generally designated with an 'ME7' or 'VRUAME7' in the subject, which is the number of the current firmware (the previous version was VRUAMDK).
For instance, this ROM claims to be 'bone stock' for the current firmware.