First, you’ll need to consult the Boss documentation for a few things.
You need a MIDI TRS > DIN adaptor to connect to the H9. There is MIDI TRS type A and B. Please consult the Boss documentation to determine which type to use.
You need to determine how the Boss outputs MIDI clock. The pedal may do this automatically, or you may have to enable MIDI clock out.
Once you have that figured out, connect the MIDI output of the Boss to the MIDI input of the H9. In H9 Control, go to Pedal Settings > MIDI Settings > MIDI Clock and set Enable MIDI Clock Input to YES.
Once you do this, you should be able to change the tempo on the Boss and the H9’s clock will follow. Turn Tempo ON for the preset you are using and the preset will sync to this tempo.