It's helpful to think of this in 3 distinct steps:
1. Assigning MIDI PC numbers to a BIAS FX preset.
2. Setting up BIAS FX to receive MIDI in your DAW.
3. Having REAPER send that command to BIAS FX.
Item #1 - Assigning MIDI PC numbers to a BIAS FX preset
1. Go to the preset name pulldown menu, and hover over the preset you want to assign a MIDI PC to.
2. Click the EDIT button.
3. In the window that pops up is where you set the MIDI PC #.
Item #2 - Setting up BIAS FX to receive MIDI in your DAW
1. Open BIAS FX on an insert on a track.
2. Open another track. This track will be used to send MIDI to BIAS FX.
3. Open this insert in the MIDI track: RealControlMIDI.
in this window, make sure that Bank/Program Select and Control Change are enabled.
(You have to leave this window open as a plugin, but we're done with this window for now)
5. Reaper menu, top, Select INSERT then select "New Midi Item".
(you may have to set the range in the timeline first for how long this should be, for example, the length of the song)
6. A MIDI item is created. Double-Click this MIDI item
7. Bottom left part of window, make sure "BANK/Program Select" is selected
8. Double click on the timeline where you want the change to occur.
9. Select a Program Number that you want to send to BIAS FX, hit OK.
Item #3. Having REAPER send the MIDI PC command to BIAS FX
1. On the MIDI track, create a SEND.
2. The SEND window comes up, select to ADD NEW SEND. (pulldown on left)
3. Select to SEND MIDI to the track that BIAS FX resides on.
Back in REAPER, hit play. The MIDI PC is sent to BIAS FX when the timeline cursor passes over the PC you've created. You should then see your preset change in BIAS FX.