I'm assuming it is turning over just not starting.
If that is the case you could start by bleeding the fuel line, that would be quick and easy and wouldn't hurt anything. Next step would be a new fuel filter (buy 2 while you are there). If neither of these work, you are draining the tank. If you end up running the bad fuel through the new filter w/o it starting, you may want to change it again after you drain the fuel.
I would not add more diesel to what is in there. If you have bad fuel, you won't be able to dilute it enough to get it decent. Where the fuel filter is, you will see a rubber line running into it from your left as you are standing on the right side of the tractor and looking at the fuel filter. You can disconnect it from there and drain it. Just be real careful as it can be a pain to get reconnected. I had to change my fuel filter assembly and ended up pulling that line off of the tank. Ended up having to disassemble the dash to put it back on. The other thing you could do is just remove the fuel filter and bowl and let it run out through the fuel filter. That is a little messier as it doesn't flow as smooth.
Good luck and let us know the outcome.