If your lid is still attached to the base you will need to remove the hinges first as the lid cannot be split when still attached to the base. For this read the sister article 'Lewmar Low/Medium Profile hatch - release hinges'.
So you now have a lid to work on.
Bear in mind that your hatch has been exposed to salt water for years and so will probably need some extra help to release the two sections of lid frame, unlike in the Lewmar video which uses a nice new lid!
So, the first thing to do is remove the screws. Often these will come out with surprising ease, but if you feel you're likely to mash the cross head, do stop and apply both heat and fluid to the area before trying again. Shocking the screw head with a drift and hammer can also help break any corrosive bond. Remove the screws and place in a suitable pot so you don't lose them. Note that we take out screws on both sides at each split, even though we want the fishplate to remain in one side. We do this as we want the easiest side to let go.
Next we apply heat (gun) and releasing fluid (eg white spirit) to the area arounmd the screw holes (fluid down the holes). The two frame halves are conected with an alloy insert we refer to as a fishplate. This will be corroded but hopefyully one side will come out in due course.
We're now ready to try to force the frame sections apart. We'll initially start with the hinge side as this is easier given you have the hinge rebates to push against. We use a modified sash clamp for this. Normally a sash clamp pulls things together, but ours is modified to push. Without this you could build a frame (eg wooden) to extand beyond each side of the lid and use G clamps on each side to have a similar effect. Don't try using a hammer as this won't work and will likely damage the frame. Constant, heavy pressure is what's required. So here we have started to separate the hinge side. When this has moved a little (5mm), leave this and move to the non-hinge side.
If you havn't already, cut away the closing seal. Once the cut is started you should be able to pull the remainder away. This will expose the alloy frame which is a better surface to push against. If the tool you are using won't sit square against the frame add in some form of spacer which does. You may (as here) want to clamp this down onto the acrylic to ensure it won't rise up on the frame when under load. If you clamp away from the pressure point the spacer should still move/rotate when under load. I'm using a metal wedge as spacer here but a piece of acrylic would do too.
So you may find that this is sufficient to get the split to move, in which case wind away. However if the split is not moving, keep some pressure on, but then see whether you can push something into the inside face of the split as shown. We have some very shallow (ie fine) metal wedges and I use this with a plastic hammer to try and initiate some movement while under pressure from the clamp. In this case it worked. Do be a little carful though as the ends of each frame section will bend/compress/mark if you get acrried away thumping the wedge. This is one reason we use the wedge on the inner side as this will be partially covered by the new closing seal.
With the non-hinge side moving apart, take this mostly apart and then return to the hinge side to extend this gap. Once the fishplates are fully out the frame should just pull apart allowing the old seal and acrylic to be removed.
Next, clean up the lid frame ready for the new seal and acrylic to be fitted.