It's an easy task. The hex head on the picture is easily visible and accessable from underneath (17 mm., I think). Have the new one ready (with the aluminum crush washer for sealing) so you can slap it right in and not get much "moter erl" on yourself. Don't over torque.
The Golden Rule
1984 300SD (bought new, sold it in 1988, bought it back 13 yrs. later)