To change an Allegiant flight, you typically need to contact Allegiant Air's customer service. Here's a general guide on how to do it:

1. **Contact Allegiant Air Customer Service**: You can call Allegiant Air's customer service number to change your flight. The customer service representative will assist you in changing your flight details.

2. **Provide Booking Information**: Have your booking information ready when you call, including your reservation number, name, and any other relevant details.

3. **Explain Your Situation**: Inform the customer service representative that you need to change your flight. Provide them with the details of your current flight and the new flight you wish to book.

4. **Check for Fees and Availability**: Be prepared for potential fees associated with changing your flight. Additionally, confirm the availability of seats on your desired new flight.

5. **Follow Instructions**: Follow any instructions provided by the customer service representative to complete the flight change process.

6. **Receive Confirmation**: Once the flight change is processed, make sure to receive confirmation of the changes made to your reservation. Double-check the details to ensure accuracy.

Remember, policies and procedures may vary, so it's always a good idea to review Allegiant Air's specific policies regarding flight changes on their website or by contacting their customer service directly.