How can I control preset change on the M350 live? What kind of footswitch can I use?
To control user preset change you need a MIDI controller. The M350 responds to MIDI program change.
The effects ON/OFF and Reverb ON/OFF can be controlled via MIDI CC's. Your MIDI Footcontroller should be able to send MIDI CC in a latching mode from switches (send values 0 = off and 127= on). A few other parameters can also be controlled via MIDI CC from a switch or a midi expression pedal. (See list in the manual)
Here are some MIDI boards examples : Roland FC300, Voodoo Lab Ground control pro, Rocktron Midimate or All access. Etc..
The pedal input on the rear of the unit is designed for a 2 switch momentary pedal. (i.e. Boss FS6U)- You can control the bypass with one switch, and the TAP tempo with the other switch.