Hi,
Welcome to TFP.
Actually the fixture itself is mounted into a thing called a Light Niche. The Nitche is fixed to the pool wall. On the back of the light nich is a conduit fitting and the conduit for the light cord goes through that fitting, thru the conduit and to the switch.
Disconnec the cord from teh light switc, remove the light fixture from the Nice and then take the whole thing out.
Not a problem to do at all. Do just the reverse to put it back in when the new liner is installed.
Edit: Tip. WHen you disconnect the light cable from teh switch, use some electrical tape and tape a pull string to the cable. The use the string to pull the cable back to the swtich when you upt the light back in.
hope this helps,