Disconnect your amplifier’s power supply.
Remove the back plate from your amplifier if you have one. Many amplifiers have a protective guard on the back panel of the amplifier to prevent damage to the tubes and sensitive electronics. Most back plates are held in place by a number of screws.
Identify the preamp tubes. Most modern amplifiers will have three preamp tubes and four power amp tubes. The preamp tubes will be much smaller and are easy to locate. If the preamp tubes are housed inside protective individual casings, push down on the casings and rotate counterclockwise. The casing will pop up and can then be removed to access the preamp tube.
Grasp the tube firmly at the base and pull it out of its slot. Make a mental note of the connector pattern on the bottom of the tube.
Insert the new tube into the slot. Match the connector pattern on the new tube to the placement you saw when you removed the old one. Push the tube firmly into place. Replace the protective casing by sliding it over the tube, pushing down and rotating it clockwise.
Replace the remaining two tubes and take the old tubes to a music shop or recycling center for proper disposal.