A few centuries ago, tradesmen who wanted to sell their wares hired servants to hold the goods up high so passing buyers could get a good view.  Those servants were call upholdsterers, because they held up the goods.

​

Later, upholdsterers were given additional duties.  They were taught to fix up the goods by draping them with new coverings, perhaps so they would look attractive to potential buyers.

​

Eventually, over time this became their main duty.  That is why , today, a person who works with textiles on furniture is called an upholsterer.  Somewhere along the line the "d" was dropped.