If it's not sliding properly, try cleaning the track well and applying silicone spray.
Make sure you use silicone spray and not WD 40 which can collect dust and impede opeartion.
what exactly is wrong with it & what kind is it different ways to get them out to fix them
Found this on line Call a qualified glazing company to look at the door. As a glazier I can tell you that there is a lot more to changing out a patio door unit than you see. Most patio doors manufactured in the last 25 years that are of the "aluminum" variety have pretty common sizes unless they are custom doors, which makes them cheaper as most suppliers of glass shops carry units "in stock" for these sizes.
When changing out a unit, we must remove the corner screws and sometimes corner keys along with the gasket. If the gasket is broken or worn, good luck finding a new one if you're not in the glazing industry. We carry about 15 different sizes. Then the unit has to go back together by hitting (with a mallet) the styles back onto the glass: if not proficient in this methodology you can break the glass and/or the new seals. Then you must align the unit, check the rollers and adjust for the new placement in the track and lubricate.
It Is far cheaper to hire someone for this type of job as it may be daunting once endeavored upon, at which point may be too late. MORE INFO> https://ask.metafilter.com/163063/Sliding-glass-door-condensation
That depends on what's wrong with it. You can clean it and lube it like the other folks suggested. There are replacement kits for the wheels and track as well. I did my mother in laws last year. She set an appointment to have some guy come install a whole new door and track (which she really couldn't afford) so I ran to the hardware store and bought new wheels. Pulled the door off the track, flipped it up onto a big table outside, and the wheels that were on it were so worn down they were square. Installed new wheels, put door back up and it's so smooth now. Saved her a couple grand
Make sure the rollers are sitting on the track properly. If not, you will need to lift the door such that it is on the track. There is usually an adjustment to lower the wheels (thus lifting the door). The house may have settled at the door location. A door company may be able to make adjustments without the cost of door replacement.