The FS308 Aerator doesn't require a key to remove the aerator.
The tip of the faucet can be unscrewed with a rubber cloth to get a grip on the faucet tip.
Once removed the aerator can be lifted out and replaced with a new one.
The photo below shows how much of the spout is removed to replace the aerator.
XP-12413 is the part number for the aerator.