How do you open the left side plate of a Revo S to get to the centrifugal brakes? The manual doesnt say how.

Pretty easy.

See the big screw on top of the reel ?

Loosen it

Then go to the non handle sideplate of the reel:

Twist it to release the sideplate.

I posted this a few days ago on the exact same questions:

All the Revos, regardless of model, open the same way:

1) Start by loosening the knurled screw at the top/front of the handle-side side plate. The screw will not come all the way out. You'll know, though, when it's loose.

2) Twist the brake-side side plate clockwise about 1/8 of the turn then lift off.

3) Once the side plate is off, you'll see the brake. They are little plastic weights on pins - 6 of them. You turn them "on" by pushing the weights up on the pin, or "off" by pushing them down on the pin. There are 6 of them and you can adjust one, any, or all of them to achieve the desired performance.

4) Once you adjust them, place the sideplate back on the reel at the same angle you took it off, then twist it counterclockwise to lock it into place.

5) Re-tighten the screw. DO NOT use a screw driver, only twist it thumb/finger tight.