The belt tensioner is spring loaded. As NevarMore indicated, you move it against the spring tension to relax the belt. Usually this is done with a wrench on the tensioner pulley bolt; some designs have a square hole in the tensioner arm to use a 3/8" or 1/2" square drive tool. I would expect the pulley bolt and alternator bolts to be metric, but it’s not out of the question that they’re SAE.
With the belt relaxed, it can be easily slipped off a pulley, after which you can release the tensioner. Be aware that the tensioner will move beyond where it was to its own stop point, since it won’t have the belt to stop it. To replace the belt, thread it onto all but one pulley, then pull the tensioner as far as you can to make room to slip the belt onto the last pulley.
Usually there’s a belt routing diagram on a sticker somewhere under the hood. If you don’t see one, make your own diagram before removing the belt.
As Berkut intimated, a squeaking noise could have caused by the belt itself or by the alternator. The WD-40 could have quieted either one. If the noise returns, you can sort it out. Try a hit of WD-40 on the ribs of the belt somewhere away from the alternator. If the noise stops, it was the belt. If the noise remains, try to hit the alternator just behind the pulley, not getting any on the belt. If the noise then stops, it was the alternator.
There’s no practical way to service alternator bearings. They can be replaced, but without some experience and correct tools it could be a major pain (or worse). I would advise getting a quality rebuilt alternator.
Your description indicates a serpentine belt (snakes in and out among several pulleys) which means it’s multi-ribbed and long, which means it’s going to cost noticeably more than 5 bucks.