How to replace the A/C belt tensioner?

It is unlikely that the tensioner is causing this noise as the belt would most likely be making this noise with the A/C on or off.

I suspect the compressor is the issue. Even though the tensioner will move with the engine vibrations it still won't move enough to be knocking.

What I would do is lift the hood and use a stethoscope to pinpoint where it is coming from. To do this you just listen to where the noise is the loudest and that is the part that is making the noise.

If it is the tensioner, you should see if move as soon as it makes the noise.

https://www.2carpros.com/articles/serpentine-belt-tensioner-replacement

However, I am thinking the compressor clutch or compressor itself is the issue.

https://www.2carpros.com/articles/replace-air-conditioner-compressor

I am attaching the process below to check the tensioner. Basically when you move it, there should be firm tension on it and not that easy to move all the way to the end of its travel.

If it move easily then the spring has failed inside the tensioner.

Please see below and let us know what you find with this. Thanks