trick is have lots of patience. don't put it off to summer.

remove the window trim, dome light, grab handles; whatever else seems to be bolted thru the headliner.

the front and back have a plastic bar running over the window, anchored by little slots. if you pull the headliner towards the window you're working on, working each corner, you should feel it release from the keepers.

next the side anchors; they are little teeth that run over the door frames between the roof. You need something long & thin to work them free. Get it to where it's just hanging down once you release all the barbs along the sides.

finally, there a metal spars the ends of which slip into points in the lining. Undo these and it will fall free, or disintegrate.

make sure the replacement has the plastic bars at the ends, the spar anchors are in roughly the right spot, and the whole thing is the right size. Helps to have two people.

Most of these liners are around $100, and are plain wrong for the car.

A decent upholsterer could probably furnish a liner & install it for around $200.

it's worth the extra expense.
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1966 122s, 1970 142s, 1974 142e... Blue is beautiful