How to upgrade from erp next 12 to erp next 15

We have currently installed erpnext 12 in our organization which is customized .i need to know that can i upgrade my current setup to erpnext 15 version on the same server or do i have to setup erp next 15 on the fresh server with customization to be made accordingly?

You MUST upgrade in stages…v12 → v13 → v14 → v15.
Make backups before you start each step, and test each version (even if it’s briefly) before proceeding to the next (otherwise you will cause problems and potential corruption