Substitute sitter?

So I have a regular customer who books weekly walks month by month. In October I confirmed a booking for every Wednesday in November, (shows up as 1,8,15,22,29). The problem is I was recently given a trip to Mexico over Thanksgiving week so I'm now unable to do the date on the 22nd. Question: Can I get a substitute walker for that date? Or how can I cancel a single day of walks on a booking that has 4 other days? I've already done one of the days so if we cancel the booking will i not get paid? Thanks in advance for the help!