"Updating" a letter of recommendation? I just realized that I forgot to mention an important detail to my professor, who already submitted his recommendation.

Is it typical to update the letter, given that the piece of information is very important?
Let me be direct here: You are already asking your professors a big favor for writing these letters for you. It is your job to give them all relevant materials up front, and if you fail in this, then you'll have to figure out a way to fix this yourself -- for example by writing to the department where you are applying, putting this information into your statement of purpose instead, or in some other way. You are overtaxing your professor's goodwill by asking him to go back, rewrite the letter, find a way to get that to the departments he sent his letters to, etc.

All of this is of course independent of the actual possibility of updating a letter of recommendation. In most cases, professors get a link to a website where they can attach a letter, and this link becomes dead either once the letter is submitted, or once the deadline for these letters passes. It may become exceedingly tedious to find a way to update a letter once that is the case.