Yeah, the pulley is like $20 at NAPA or O'Reilly'*. Left hand thread on the nut - zip zip - yer done.
Now, if the arm is broken or the spring is broken or weak and you do need to replace the whole deal, to answer your question:
The steel elbow joints can be a little tricky to get out of the aluminum bores in the tensioner assembly. I put a little penetrating oil in mine, removed the little 5/16" nut that holds the "L" onto the casting and broke the corrosion loose by twisting on the heavy vertical steel part of the "L." Be careful not to use the horizontal tube and barb for the heater hose as the lever for twisting -the metal is very thin and will easily bend and distort. The vertical potion of the "L" is very stout and will allow a pair of channel locks with no problem. You will need new o-rings, of course.