/n/ → /m/
The nasal alveolar /n/ becomes a nasal bilabial /m/ when it’s followed by a bilabial sound, such as /p/, /b/ or /m/, because of assimilation. The change only affects the place of articulation, but the manner of articulation (nasal) remain the same.
/n/ (followed by /p/, /b/ or /m/) becomes /m/
And here is an example by a very well-spoken native speaker:
Facebook has been the subject of, over the past few weeks, a series of damning revelations in The Wall Street Journal called The Facebook Files, which cover everything from changes in Facebook’s algorithm, allegedly inciting more sort of divisive content, to Instagram being damaging for teen mental health (Dan Milmo, Today in focus, The Guardian).
Notice that here teen /tiːn/ sounds like team /tiːm/ because of the assimilation.