Update the Item through CSV Import to add quantity item pricing
Navigate to Setup > Import/Export > Import CSV Records
Select Items in the Import Type field
Select Record Type
Select the CSV File ( Of the item pricing details need to update)
Click Next
Select Update in Data Handling
Click Advanced Options
Put a check mark on Overwrite Sublists
In the mapping area
Item Pricing 1 : Price
Item Pricing 1 : Price Level
Item Pricing 1 : Quantity
Item Pricing 1 : Currency
( If more than 1 pricing level then follow the below steps also)
Item Pricing 2 : Price
Item Pricing 2 : Price Level
Item Pricing 2 : Quantity
Item Pricing 2 : Currency
Then Save it