In English it's more natural to say she changes (her) identity, singular.
להחליף also means exchange, right?
Correct. To exchange
Maybe she is an administrator in a witness protection program.
"Identity" as in she changes her character, gender or she pretends/acts as someone else?
Well, it means the latter, to pretend to be someone else. זֶהוּת is a curious modern word formation, using זֶה "this" and the suffix for abstract concepts ־וּת
Or - could it be that she is a system administrator changing the identities of the SW users? Guessing that from the fact that it is not "הזהויות שלה".
hee machalifa zehuyot
הִיא מַחֲלִיפָה זֶהוּיוֹת
Could she be logging into her second Duolingo account?
Of course not. Would "identity" ever be used this way? In any language?