If you are lucky it s in good shape and either made from 2x12 s or even better a glulam beam.
Making garage door opening taller.
Before removing the header you will have to support the roof above it.
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.
Easy to do and it can solve alot of problems.
Usually 4 or 8 feet.
That is the side where your truck is to go you can build out on the back side.
To put it into perspective if the garage door is let s say 8ft tall make the height of your opening 8ft 1 5 inches tall.
This means building a wall that spans the entire distance of the opening and is strong enough not to move until you are done.
If you have the room on the back side of your garage you can make your garage into what is called a jack jill garage.
I removed some siding and and re did the trim on the top of the door and then recovered every thing with aluminum and installed new rubber garage door sill around the opening.