To adjust any belt on a 240 you loosen all of the 13 mm nuts and bolts first. The AC compressor has an easy to miss nut that is sort of hidden on the top side of the compressor that folks miss all the time.

Take you hand and feel along the back of the bracket between the power steering pump and the AC. When you fell an oval headed bolt you are almost there. On the other side of that is a 13mm nut you just need to loosen. Then you loosen the lower nut and bolts, do not take them off. Just loosen the nuts is all you need to do. Then you turn the long 10mm bolt CW until the nut is tight. Only then do you retighten the nuts and bolts on the compressor. I would loose the power steering pump before doing that, you just loosen the 13mm nut on the lower bracket and loosen the long 10mm nut by turning it CCW. Then after you get the AC belt tight, crank the long 10mm bolt Cw until that belt is tight. Tighten the 13mm and nuts on the power steering pump and you should be done.

Good Luck,

Paul