Changing tubes is easy - definitely something you can do yourself. There are lots of different brands and you will get a lot of different recommendations, many are down to personal taste. I don't play hard rock or metal so I'll just run through what to do.
I would change the power tubes first - the 6L6s - these are the ones that wear out most quickly and lose power and clarity when they do. If you use Mesa-branded tubes, it's simply a question of pulling the old ones out and putting the new ones in, as Mesa tubes are pre-selected for correct bias - and the amps are not adjustable, unless modded. To get them out, push down the two metal 'bear trap' clips with one hand, and gently pull the tube out with the other, rocking it from side to side a little - but not too much, or you'll break off the plastic center locator pin. Make sure the locator on the new tubes is lined up before you push them in. Other brands will work too, eg Groove Tubes if you stick to the middle rating bands (4-7).
If this doesn't fix it, you may need to change the preamp tubes (12AX7s - the small ones), or some of them. There should be five preamp tubes - one is under a metal cover. Any 12AX7s will work, and they do not need biasing. The most likely to be worn out and giving the sound you're getting are the 'phase inverter', and the first gain stage (called 'V1'). These are the two at opposite ends of the row of tubes, V1 is the one under the metal cover, which turns to remove it. If that still doesn't cure it, change the others as well. Preamp tubes also sometimes go dull and weak, but not as often as power tubes. Preamp tubes can sometimes be a bit hard to line up with their sockets (they also have a locator, it's the gap in the ring of pins), and you need to make sure they're lined up before you push them in or you'll bend the pins and may crack the glass.
The rectifier tubes (5U4) either work or don't work, they don't seem to wear out slowly. If one has gone it may make the amp seem too quiet and a bit dull. Test it by switching the amp to Solid State rectifier - if it suddenly gets *massively* louder and brighter, you may have a rectifier fault. (It should get a *little* louder and brighter normally.) If you think there may be a fault, try removing one of the rectifier tubes and try the amp in Vacuum Tube - if it works at all, the tube in the amp is fine. Now try the other tube - making sure that the locator pin is present and lined up correctly, as with the power tubes.
None of this is dangerous, there are no high voltages outside the chassis - but you should make sure that the amp is turned off, unplugged from the wall, and for maximum safety that the standby switch is in the 'play' position, as this will drain off any remaining charge inside. The real danger is burning your hands - power tubes and rectifiers get *hot*, and take quite a long time to cool down - and cutting yourself if a tube breaks in your hand, which is rare but can happen.
Keep any tubes you take out that are not definitely failed - you can use them as spares in an emergency even if they're not performing as well as they should - at least until you're quite sure the new tubes are OK, the initial failure rate on new tubes is higher than it is after they've been 'burned in' for a while. I would keep them (and a couple of spare fuses of the right rating) with you for gigs anyway, since it could get you out of a tight spot.
Good luck, and hopefully you won't need to pay for having tubes changed again!
