How to modify all items at once.?
what do you need exactly to modify ?
Tax rate, prices and other options in the item panel
As you already have them in ERPnext you can export them with data export tool while checking download with data too all fields, do your update on excel which is so easy then import back the excel file using data import tool while selecting update records and do not check skip rows with error and check don’t create any new records if you don’t want to add new records either.
There is no other way to
To be honest with you , i just know this way , not quite sure if there is another way ( though in technology there are always alternatives ) it might be a way that i don’t know , just saying .
Thanks 
for your help
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Things like this can also be done via the API.
You can directly access the API, or use frappe-client