Once you have the back or cover removed from the amplifier/radio/record player you are changing a valve in it's easy to change. First, the safety warnings;
Unplug the unit from the mains before removing any covers.
Don't try to change the valve when the unit has been recently switched on... valves get hot enough to burn you!
Try not to touch anything inside the unit except the valve you are changing as some parts can hold high voltages even when it has been switched off for some time.
Now for the easy bit.... hold the valve you are changing between two fingers and your thumb and rocking it slightly to free its pins, pull it firmly straight up and it will unplug. You will see that the pins form a circle with a gap in it where there is no pin.
Take a look at the socket to see where the gap with no pin socket is and taking the new valve, line it up correctly with the gap and again rocking it around slightly (only a few degrees) push the new valve in until it is firmly seated. Replace back cover, switch on.... all done!