How can I bulk edit my accounts data - 20000 accounts

How can I bulk edit my accounts data - 20000 accounts

I need to be able to see all of my accounts in an spreadsheet view - then edit row by row a custom field.

I have the Advanced Reporting access where I can see all of my Accounts data but cannot edit/save directly via Zoho Advanced Reports.

What would be the the easiest way to do this bulk uploading?