Sometimes garage doors need to be made larger because a larger vehicle like a truck or RV has been purchased and needs to be housed in the garage itself. There are a number of different ways to do this, most very expensive and probably not worth the money.
Because garage doors are usually sized to snugly fit the dimensions of the garage itself, making the door larger usually means making the garage larger. One way to do it would be to get wall extensions for the entire the garage and then cut the roof of the garage off, lifting it up to the height you want it. Then new wall sections need to be added to the taller sections between the roof and original wall. Once this has been done effectively, which is a rather major construction project in and of itself, a new garage door should be purchased, sized properly to fit the enlarged garage door.
Different Types of Doors
Sometimes all that is needed for the larger vehicle to fit inside the garage is 1 foot or so of headroom, which is blocked by where the garage door hangs when it is open. This is the easiest and cheapest solution for enlarging a garage door because the hanging garage door can just be removed entirely. The replacement garage door should be one that hangs above the front of the garage and lowers down from the outside, so it doesn't affect the height of the garage. An alternative would be a pocket garage door that slides sideways on tracks from within the garage door itself, so that again the height or width are not effected.
Not Worth It
Unless you have a lot of extra money at your disposal or bigger plans for your garage in general, going through the construction simply to enlarge your garage door is probably not worth it in the long run. Although the door can be enlarged, it's a lot of hassle just to be able to pull a larger vehicle indoors. Some alternative suggestions would be to totally rebuild the garage with more features than just a larger door or to find some other way to protect the vehicle, e.g., with tarps or covers, whenever it is parked in your driveway or in front of your house.