@Rafael156
The easiest for you now would be to ask the guests to pay the extra nights outside the system, but better call/contact airbnb and explain the situation - they might be able to help with the technical side of the matter, but it may take time. The good thing is that they may reverse your host cancellation and penalties that follow, so I do suggest you make the effort. Never-ever cancel on behalf of a guest - read up on host cancellations and their penalties.
For future reference, use the changing tool for altering reservations instead of making two separate bookings.
Look up the communtiy help guides under the discuss tab at the top of the forum.