By default, the unique prefix number will be generated for your account. You can able to change the Invoice prefix number:

1. Go to Settings from the main menu and select Point of sale under Business settings.

2. Change the Prefix number by removing the existing prefix number and Save.

Note: The new prefix number will reflect only for the bills generated after the changes are done. For the old invoices, the default prefix number only be there that can't be changed.