It is possible to expire the listing before its actual expiration date. However, it is suggested to be sure before doing so, as this cannot be reversed.
Once the listing has been marked as expired, it can't be activated again unless with a new listing altogether. Expiration of the listing will take place not immediately but on the same day. E.g., you requested to expire the listing today in the morning. The date will be updated on the MLS, and the listing will expire within 24-48 business hours.