p39-40 of the manual explain how to assign channels/mixes to a mute group:
Select your FX send. Press the routing button. Go over to the Mute/DCA Group Assign tab. Assign the selected FX send to Mute Group 1. Rinse and repeat for other FX sends.
What’s probably quicker is to press the routing button, press the Fn button and go over to the Mute Group Assign tab, press the Mute Group 1 Tab. This allows you to assign many sources at once to a single mute group. Assign all your FX sends to Mute Group 1.
p59 of the manual describes how to set a softkey as a mutegroup master:
Setup>Control>Softkeys. By default they’re already assigned to mute masters 1-4, but you may have changed them to things like tap tempo, scene recall etc. Anyway, ensure that at least one of them is assigned as Mute Group 1’s master. Once set up, you can press that softkey to simultaneously mute all your FX sends.
NB you could do the above with a DCA instead of a mute group if you wanted fader control as well.
HTH