Changing the bulb is easy... just pull... depending on how old your Sartek lighthead is makes a big difference though.
FWIW the new bulb comes with changing instructions... (well mine did anyway)
The new lightheads are a nice simple unscrew job and replacements bulbs are easy to get, if you have the old style circlip and glass fronted one then firstly it is a PITA to get apart and secondly not worth it because I couldn't get a new bulb for love nor money
I bought a new head
.... My advice to anyone with a working bulb is to flog the whole thing and buy a new torch, one which you can get bulbs for.
BEN