You can bulk import product attributes or item specifics into your products and then use them within listings. You can also follow the same process to bulk remove attributes from the product by specifying an empty value for those attributes that you want to remove from a product.
Please create a CSV file with following columns and upload this file in Bulk Action section /General CSV file import type.
e.g of a CSV header fields are
sku,product attribute:Colour, product attribute:Size,product attribute:Custom Attribute
If you would like to remove 'Custom Attribute' attribute from the product, please leave the value empty.
Please download a sample file below