If you have not utilize this list as the Data Source for an Email Template, you can replace the file. If the Imported List has been chosen as the Data Source for an Email Template, you will not be able to replace the file, and you will need to create a brand new Email Template.

To replace a list not previously used in a Template
  1. Navigate to Email > Lists
  2. Click the pencil icon next to this list
  3. Click Replace This File
  4. Click Choose File
  5. Choose your file
  6. Click Next
  7. Map the Email Address and Display Name
  8. Click Save
To update a list previously used in a Template
Create a new Imported list

Additional Resources
Cannot select or change the recipient list for an email message, or the merge field button is missing (includes video demo)