Navigate to 'Raw Data Upload' where you will find 'Add/Update Customer Items'.
Use report #21 "Item Listing" to get a list of all your retailer skus (customer items). Report 21 will export the data in the column order you need for 'Add/Update Customer Items'. Simply make the updates you want to make on your items, then copy the data and paste it into the white box where it says "Enter CSV Data Here". Be sure to copy all the columns, including all 24 category columns, even if you are not using every column.
You can update existing items, and enter new items all in the same paste.