If you are replacing upholstry with original replacement parts from the original airframe manufacturer, it's a repair, and no approval is required. Other than that, it's probably a modification, and a modification approval is required. I know of several aircraft upholstery shops which were shut down, because they chose to not obtain design approval for interiors they did on small aircraft, even though the work was excellent. (and would have been easily approvable, had the sought approval)
You've got to be so careful about the use of the word repair verus replacement. If you are doing a straight replacement then there is no repair as such. If you drill of a panel and obtain identical p/n and rivet it back on it's a replacement. However, if there is a scheme in the SRM or AC43-13 for fixing a problem then it's a repair.