Apple watch series 3 update

Trying to update watch, but error message tells me I don't have enough storage to do so. The watch is 2 months old and shows 4.2 GB used of 4.8 capacity. I'm using 142.6 mb of app space, of 639 mb available space. The update is 48 mb, and says it needs 866 mb of space. Not real computer literate, but numbers do not add up. Confused???