Utmost means of the greatest degree or most extreme. For example:
Success in anything is through happiness. Being happy is of the utmost importance. (Maharishi Mahesh Yogi)
Upmost means at the highest or most upward position. For example:
Take the upmost book from the pile.
Utmost or Upmost?
The words utmost and upmost are often confused because they sound similar and their meanings are close. However, they are not interchangeable.
The adjectiveutmost 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. (Carolina Herrera)
I try to be me to the utmost. (Donald Glover)
(This is an example of utmost being used as a noun.)
The adjectiveupmost 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.
Upmost Importance or Utmost Importance?
Writers are often confused whether to write utmost importance or upmost importance. The correct version is utmost importance as this means importance to the greatest degree. Remember, the word upmost is reserved for the highest physical location (as opposed to the greatest degree).