Try Effect > Low Pass Filter… or try a de-esser plug-in like the de-esser from https://forum.audacityteam.org/t/updated-de-clicker-and-new-de-esser-for-speech/34283/1 (Nyquist plug-in) or Lisp (VST plug-in).
See here for how to install Nyquist plug-ins: Installing Nyquist Plug-ins - Audacity Wiki .
See here for how to install VST plug-ins: How do I install VST plug-ins? .
Gale