Verify that the tubes in the amp need to be changed. Signs that your tubes need to be changed include frequent blown fuses, a significant volume drop in the sound produced by the amplifier, high-pitched whistling, or a low-pitched rumbling noise during play. The high-pitched sound usually indicates the pre-amp tubes are failing while the lower sounds mean the larger, power tubes are beginning to fail.
Turn off the amp, disconnect from power source and allow the amp to cool before continuing. Amplifier tubes can get extremely hot and any time you are working on electrical appliances, you should unplug them.
Use a cloth to remove the tube by carefully pulling it up and out from its socket. It is important to avoid having the oils from your hands damage tubes you are not replacing. Remove each tube in the same gentle manner. After removing all the tubes, note whether there are any burn marks in the sockets. If there are, this can be a sign of a more serious problem with the amplifier and you should contact a professional service person.
Take a sample tube to a music/instrument store and purchase the same type of tube. Using the same tube as recommended by the manufacturer assures that your amp is working to its fullest potential.
Insert the new tubes into place, simply by switching them with the old ones. Replace all of the tubes in a section at once. If you fail to do so, then the old tubes will draw extra power from the new ones, causing them to die faster.