Try the De-Esser in invideo Editor’s Effects > Audio library. De-essing targets excessive sibilance: the sharp “s” and similar sounds that can stand out in speech.
Listen to a passage containing both sibilant words and ordinary dialogue while adjusting the treatment. Reduce the harshness without making speech dull or altering the speaker’s articulation. Check the result at a comparable listening level and in the full mix. The useful amount depends on the voice and recording.