I'm no MIDI guru, but I use the 49-key version of that same keyboard.
Keep in mind that your actual "instrument" is a Virtual Instrument (VSTi) running in your DAW software (
Studio One). Your MIDI keyboard just sends out MIDI data on one of many channels.
I haven't used Studio One, but assuming that it's like most other DAW software, you have a track where you've loaded your VSTi as an insert effect. For an input source, you would select your Xboard25, and pick a channel. Channel 1, for example.
To switch instruments, you'd need to create another track, load another VSTi on it, set the input source to your XBoard25, and select a different input channel. Channel 2, for example.
Now on your XBoard, you can change between channels 1 and 2, and this will switch between the 2 virtual instruments in your DAW.
I'm an idiot when it comes to MIDI (midiot?) so there may be much simpler ways to accomplish this.