I have the Boss FS6. But haven't used it for a while. Can you describe how you set it up exactly for the H9?
In H9 Control you'll want to go into Pedal Settings / General Settings / Use Expression Pedal, Aux Switch or Both, and set it to Aux Switch.
And then go to Pedal Settings / Aux Switch Mappings and assign the aux switch buttons either to the "Increment and load preset" and "Decrement and load preset" functions or the "Increment preset" and "Decrement preset" functions based on whether or not you want the preset to automatically load.
Also are there some tips to switch presets fast without an aux switch? So just the H9l. Most of the times I want to bring as little gear as possible to a gig.
Well, a lot of this comes down to clever organization of your preset lists. I know someone for example that saves the same delay preset to every other preset position because he never wants to be more than one button press away from that delay.
You can also limit the preset range to whatever you want. So, if you're only using 5 presets in a set, it's better to limit the range to those five presets so that you can click through and wrap around to the beginning again.
And also pressing the big black wheel reverses the direction that the preset switch is going in. So, you can switch it from incrementing to decrementing for example. Some people find it easy to hit that wheel with their foot; other people not so much. I guess it depends partly on what shoes you're wearing.