Understand from CRM i have 2 times backup per month, before I proceed to do so, would like to know how's the back up putting back to the sys and what's being backuped and where will the back up saved.  Locally in my server or cloud or other placed I choose, please give more details.

"Export your Zoho CRM data for your backup. You get 2 free backups per month, for every additional backups $10 will be charged."

once back up done, we would like to delete old invoices / sales order / PO from sys, in case we need to retrieve a particular PO / Sales order, is it's possible for us to do so, if yes, how.