@Shelley159 mentioned how you can request a change in dates. However, if the date you want to change to is already booked, or otherwise not available, then you won't be able to do that. When you request the change, the Host will see it.
You can also message the Host and explain the issue.
I would say message the host in case they blocked the calendar and it may still be available. There are times when guests ask me for a change and I can do it at my end as a host, but guests can be limited, in case there is a block on the calendar. But yes, if there is another guest already booked, not much you or the host can do.