I don't know why you have decided to change them, since they pretty much last forever, but here goes.

Unplug the amp, and br sure it has had time to cool off. There are capacitors inside the chassis that can hold potentially [u]deadly[/u] voltage for a long time, so don't go there.

Be gentle with them. The pins that plug into the socket are not very strong. They're more like pieces of wire, actually. Rock the tube back and forth, while pulling on the bottle.

If you look at the bottom of a tube, you will see that the pins are arranged in a circle, with a gap. The tube socket has holes in the same pattern. When you plug a tube into the socket, you have to be sure to match the gap in the circle of tube pins to the 'missing' hole in the socket.

Rock the tube gently back and forth as you push it into place in the socket.

All done! Unlike the tubes in the power stage, preamp tubes don't require bias adjustment when you replace them.