Amps with 4 power tubes working in a push-pull configuration have the tubes working as pairs. If you pull one tube from each pair, you reduce the power of the amp by 50%. However, due to the way we hear, this isn't much of a volume reduction. You need a reduction in power by 10 times to sound half as loud, so a 100W amp is twice as loud as a 10W amp. But if you want to try, here's how.
As long as the pairs of tubes that are working together are side by side (that's all I've ever seen), you can pull:
the 2 outer tubes
the 2 inner tubes
any non-adjacent pair of tubes.
Then you need to adjust the load that the amp is driving. If you have a two speaker cabinet, like a Twin Reverb, disconnect one speaker. This will change the load from 4 ohms to 8 ohms, and the amp will be happy. If you have an impedance selector on the back of your amp, set it to half of the speaker impedance. If you use an 8 ohm cabinet, set the selector to 4 ohms.
Most tube amps can run with an impedance mismatch. When you use the external speaker jack on an older Fender amp, you are doing this. So don't worry too much about it if you only have one speaker or don't have an impedance selector.
If you intend to run the amp this way for a while, consider rebiasing because now only 2 tubes are pulling current where 4 tubes where before, and the bias may be off a bit.