The words utmost and upmost are often confused because they sound similar and their meanings are close. However, they are not interchangeable.

Utmost
The adjective utmost means of the greatest degree or most extreme. Utmost is a superlative. (Although rare, it can also be used as a noun.)

Attention to detail is of utmost importance when you want to look good.
 I try to be me to the utmost.
 
Upmost
The adjective upmost means located at the top, highest, or most upward position. The word upmost is far rarer than utmost (evidence). (NB: Upmost is mostly seen when mistakenly used for utmost.)

I placed the trophy on the upmost shelf.
 The sweetest fruit is found on the upmost branch.
 
Source: grammar-monster.com