Hi, Dirk!
You have already received a lot of good tip!
Here is my suggestion of a walk-through, assuming you have the back cover and the changekey. And please, DO ALL OF THIS WITH THE DOOR OPEN!
Dial CCW(counterclockwise) until you have picked up all the wheels.
Stop when the #3 wheel/gate is under the fence.
Read your 3rd digit of the combination off the dial. Disregard the other wheels/digits for now.
Put the back-cover on.
Continue dialing CCW until you see the square change-socket(somebody please help with correct term..
)through the changekey-hole, AND HOLD THE DIAL STEADY WITH YOUR THUMB WHEN YOU SEE THE SQUARE.
Put a mark on the dial ring where your #3 digit is. THIS IS YOUR CHANGE MARK
Now, take the cover off again. Align all the gates under the fence(you can also read your "old" combination now, if you're curious...)
Put your changekey through the hole in the back-cover. Make sure to point it in the correct direction... Insert the key in the square hole. Make sure you get through all 3 wheels, and PULL THEM CW(you are now working from the rear..)until the tip of your key fits into its hole in the lockcase, and the back-cover fits onto the lockcase. Insert screws. This procedure is a little fiddly, and you might have to attempt it a couple of times.
You should now be in the position where :
All your wheels are aligned in the change-position
You are ready to turn the changekey CW to operate the clutch-mechanism that frees the wheels
and
enter your new combination using your very personal change-mark.
Remember to stop on your last digit, hold the dial with your thumb while you turn the changekey CCW, and extract it.
TEST YOUR NEW COMBINATION WITH THE DOOR OPEN, AT LEAST TWO TIMES
By now, everything should be working just fine.
By the way, I have only seen that dial on a Melsmetall safe, with a LaGard lock previously. It is possible, that it did not leave the factory with a S&G...
Brgds,
Ed