got it!
That's a easy tank to change. Mix up 1/2 of the total water, new, ahead of time.
Get some coolers, old fish boxes, etc to hold the live rock and a trash can, or something like that to hold the water and fish.
Take 1/2 the water out, into the trash can, put fish in can.
Take the rock and dunk, swish, shake and drain it good in the tank - clean it - then put in cooler.
Throw the 1/2 of the water left in the tank out.
Throw gravel out.
Replace with washed, cleaned, whatever.
Take 1/2 of the water out of the trash can (old water saved) add to tank.
Fill trash can up slowly with new water - acclimate fish.
remove and replace water in trash can - continue acclimating fish - as you add water back to tank and arrange rocks back.