If you have a single battery, you can get at it by sliding the seat forward, and lifting the cover. If you have two batteries, the front one is the one you need to disconnect, so you might find it a bit awkward to get at without removing the rear one, unless you take out the seat.
Always disconnect the earth wire first as touching metal with a spanner is always a possibility. As I said, you might find it easier to remove the earth bolt on the outside front of the seat base, door side, this will save having to get at the battery itself