From 2F, you just follow the signs for 2E (they say something like 2E, Halls K, L, M) - they're bright yellow and honestly pretty hard to miss. You'll walk through a separated ceiling tunnel from 2F to 2E, over the baggage claim area, and then down a set of stairs to passport control. After passport control, follow the signs for hall K, L, or M, depending on where your flight is leaving from. You won't pass through security again, but if your flight leaves from L or M, you may have to take the shuttle train (I can't remember if the 2E one has a color) that goes from K to L/M. The passport control area is downstairs from Hall K, so if you're in Hall K, you'll just take a set of escalators upstairs and you'll be in the terminal - otherwise, you'll follow the signs to the shuttle train to L/M.

I know you mentioned Hall M, but there's no way to know in advance which hall your DL flight to ATL will leave from - just look at the DL app or the departures screens at CDG when you land from GVA. Pre-COVID, Hall M used to close at 2 pm, although I have no idea what the latest schedule is for when M is/isn't used.

CDG is an admitted maze, but as long as you're careful about following the signs, it's really not a big deal. 2:25 should be more than enough time - I did 1:30 a few weeks ago from OLB en route to BOS, coming into 2G (which is a total hike away from the K/L/M), and I still had 15 mins to kill in the AF lounge before my flight. Passport control is the wild card, but 2:25 should be totally fine.