When you need an adjective meaning (1) of the highest or greatest degree or (2) most extreme, the word is utmost. When you need an adjective meaning situated at the top, highest, or most upward position, the word you’re looking for is upmost. The latter is an old, almost archaic word that now mostly appears where writers obviously mean utmost.
Clearly, the Panthers are doing their utmost to entice people to visit the rink. [Regina Leader-Post]
You can feel that the developers thoroughly enjoyed their work and each step was made with the utmost care. [Battle Creek Enquirer]
After reversing the door, the operator shall return the door to, and stop at, the full upmost position. [Code of Federal Regulations, LSA]