An analog gage is pretty easy to replace. Pull the gage panel, remove the wires and gage. You could probably test it by putting a 9 volt battery on the contacts. The electronic gage I believe is just unplug the old one.
The sending unit depends on if you can actually get access to it. If you can get access to it, remove the wire, unscrew it and replace. Sometimes you have to get imaginative to figure out how to reach them. There are videos online for several machines. I just replaced the sending unit in my A300. The manual says to remove the engine and pump but I was able to do it with the engine and pump left in.