You have some gears at the bottom of the shaft. There is a vertical rotating shaft going down the leg of the engine with a gear on the end of it, this meshes with a gear at 90 degrees to it to turn the vertical rotating shaft into a horizontal rotating shaft. There is a big screw on the bulge behind the prop on my engine to drain the oil that surrounds those gears. You unscrew it – drain the oil, invert the engine and refill. You need to use the right sort of gearbox oil. Check the manual or online.
Check what you get for £75 -I suspect they change the spark plug, and the gear oil and grease moving parts. Probably not much more than that.
Changing the plug, gearbox oil and checking the impeller is easy stuff. Taking the head off is (I admit) a bit advanced so you have to make your own choice there and definitely don;t do it unless you are absolutely sure you need to. My problem is that mine seems has needed this a couple of times so I need to do it myself or it would be too expense. Check the manual or online for the right sort of spark plug – mine needs one that is designed to run either hotter or colder than normal engines – I can never remember which way round it is.