OK, lots of misinformation here.
Those sprinklers can be installed with the supply line from the side or the bottom.
The easiest way for you to remove this will be to just cut the supply line as close to the sprinkler as possible, then replace it, using a new fitting.
Alternatively, you COULD dig all around the existing sprinkler, giving yourself enough room to turn the entire housing...
I have 3 Rainbird models similar to this in my back yard. They're not difficult.
What's wrong with this one? There isn't much to go wrong with them...usually they get grass inside them which stops the mechanism from working properly.
BTW, when you replace it, if you go with a similar model, your best method will be to connect the threaded adapter to the sprinkler housing, (use a bit of thread tape) then glue the adapter to the PVC pipe. Otherwise, you're still going to have to dig a big hole to allow you to turn the entire housing.