How do you replace a fuse in a tube amp?

The amp will have probably a mains fuse on the power input circuit and other fuses on the dc power supplies to the circuits. There might also be fuses on the audio output circuits. The fuses will be in conventional holders.

You have to find the fuses, take them out and look at them. In a good fuse you can often see the thin wire running through inside, and if the fuse has blow you can see the damaged wire.

Then having identified the faulty fuse you can look on the internet for a replacement of the right voltage and current.