There are three things you can do to an aircraft: Maintenance, where you unbolt something, and bolt the same thing back on, Repair/replacement where you bolt on a different one of the same thing, in place of the damaged one you removed, or Modification, where you put something different in place of what you removed.
With few execptions, modifications require design approval. This could be an STC you bought from the holder, or an approval you sought for yourself, through your authority's system for that.
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)
Caution, when having such work done!
Pilot DAR