So I was driving to take my G/F to her stupid hair appointment. When all of a sudden the "check charging system" notification went on. When I got home I tested the battery with the multimeter and it was good. But when I turned the car on with the A/C, headlights, rear window heater, etc. The reading dropped, so I'm suspecting a bad alternator. I checked the connections and they seem fine.

Does anyone here have experience on changing the alternator or is there anything I missed to check? Taking a look at the serpentine belt, I'm kinda lost on how to relieve the tension. Any help is appreciated. Thanks.