The back cover of the Redmi Note 9 is held in place by plastic clips and adhesive. It might require some force to release the plastic clips.

Prepare an iOpener and apply it to the back cover for at least two minutes to loosen the adhesive underneath.

A hair dryer, heat gun, or hot plate may also be used, but be careful not to overheat the phone—the display and internal battery are both susceptible to heat damage.