Hi Jodi,
If it is a previous guest, then you must have his email address or simply can contact him via previous messaging on the Airbnb platform.
Ideally would be for both of you to be on line at the same time so he can book whilst you release the calendar.
The other is that you remove your "instant book" and release the space - that way you can select who is taking this space. Nevertheless this second option might make you loose bookings, hence first option is ideal.
I have done it several times, just need synchronisation with guests.
Happy Hosting!