I have seen the words utmost and upmost confused in pieces of writing on many occasions. This isn’t surprising. What tends to happen is writers just go with the word that sounds most appropriate, but the fact is these two words mean entirely different things.
Upmost comes from the Middle English word “utmest”, which is part of the Old English language. Ut meaning “out” and mest meaning “most”. Utmost, on the other hand is used in the same context as outermost, and means the farthest point. It is used to indicate an extreme. For example; “The utmost extent of the law”.
Upmost is a form of uppermost, meaning highest in rank or prominent position of power. Upmost isn’t common in American English, although is still used fairly often in British English.
Be careful not to confuse utmost (degree) with upmost (position). If you decide that upmost is the word you want, then consider using uppermost, which has become far more common in recent times.
Examples of using uppermost:
Uppermost in her mind were the risks of getting into the relationship knowing his past.
The worst crooks are often found in the uppermost echelons of society.
I hope that has solved this word dilemma for you.