Gently list the lens back off. It is now completely separate from the lens barrel.

On reassembly, ensure that the ribbon from the barrel is correctly tucked into its pocket before offering up the back.

On reassembling the back, make sure that it's correctly aligned with the barrel. It may take a fair amount of jiggling to get it to go fully home, and if you didn't fully close the lens before starting it may be almost impossible.

On reassembly, ensure that the ribbon from the barrel which you tucked into its pocket is correctly positioned and not pinched before inserting the screws.