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You would need to shore up an structure above (ceiling, roof, etc.) with a temporary stud wall. Then you would take out the header that is above the existing opening and the studs on either side of the opening. Cut any studs above the opening so that the new header could be placed in a foot higher than the previous header. Install a new header and posts, sized by an architect or structural engineer, that are capable of spanning the new opening and carrying any loads that may be above it. Lastly, install the new garage door and do the finish work.