We get asked how often do I need to change my valves? This is subject to personal taste really as long as they've not shorted or failed otherwise physically. When valves age they tend to give a softer sound sounding like the speaker is wrapped in carpet slightly distorted and muffled. We generally say if you play every day for a few hours then 2 years is pretty good. If you are a bedroom player never thrashing it you could quite happily get 4-5 years without issue.
Replacing the valves is straightforward, relatively... the smaller tubes often marked 12AX7 or ECC81/3 can simply be swapped out without any worry or need to rebias. It's important
to notice where the missing pin is as this is the alignment to ensure the right pin is doing the right function. The output valves often the larger ones that generally come in sets of 2,4,6 should be replaced in associated quantities as they work together to push and pull like a tug of war. In most amps they need to be biased which involves measuring the high voltage often abbreviated to HT, measuring of the idle cathode current draw and some ohms law to work out the current, with the known value of resistance of the cathode shunt resistor, and voltage.
We have tables that show the dissipating current curves vers plate voltages and work out the current from here to set the biasing. It's not something we recommend to do outside of a controlled workshop you can easily be exposed to voltages in excess of 500V
We also fit an electronic automatic valve protection system and self biasing adjustment known as Tubesync you can read more about it in the link.