You could delete all contacts in your account however, we DO NOT recommend this.
Deleting ALL contacts will remove ALL communication History including the most important information - Unsubscribed Status.
If a contact in your account has previously Opted Out, this information is held and the contact will never get another message from the SMS platform.
When you import a new file after deleting all contacts, ALL the contacts in the fresh import list including the contacts that the Opted Out will also be imported as Active since the platform now does not have any OptOut information, potentially you ending up with sending them messages again.
If you wish to import a file with new info, just import the file and the SMS platform will ignore duplicates and maintain the Opted Out contacts history.