How do I replace a slider door?

Unfortunately your subfloor is a particle board underlayment that has been damaged by water. Be prepared to cut out and replace a section of the subfloor. The new door needs to sealed at the bottom and have appropriate flashing installed.

Find the brand of slider and go to there website and look at install instructions. There's usually 3 or 4 screws on each side and some along the top (number depends on size) to hold it into the hole framed for it. Most times you can just unscrew to door and remove it and put in another one. They can be heavy and awkward to handle. It will take at least 2 people. Make sure you have the new door there before removing the old one. Your flooring will probably need to be replaced as well.

Where is the water coming from? Do you need new gutters? Is the ground slanted toward the house? We had a problem with a gutter overflowing and the water would sheet down against the glass door so we were mopping up water in our sunroom every time it rained. Luckily that floor is tile. Replacing the gutter solved the problem and the door hasn't leaked since. My friend's floor smoothed back out once the leak was fixed and the floor dried so maybe you should get a well sealed door or fix the source of the water first and wait a few months on the floor just in case.