The tiny screw. If you don't already have a screwdriver for this, look for an eyeglass repair kit in a pharmacy/drugstore/convenience/truckstop. Loosen it until the head is just flush with the outer surface of the focus ring. These screws are a pain to find and reinstall if you remove them completely.
The four black screws here, outside the front element and recessed, hold the focus ring in the aluminum focus helix are the ones to tighten. They look like JIS screws. If you don't have a JIS screwdriver already, you can probably get by with a Phillips #00 because you are tightening, not loosening. Just be careful - the Phillips only kind of fits so you can't apply too much force or have the screwdriver off perpendicular. A Phillips #0 will not work and most likely ruin the screw for any future use.