Usually we are using the import template ITF (or a simplified version of it) to operate mass change on product-site records.

Where do I find the ITF (simplified version of it)

Setup > Usage > Imports / exports > Import/export templates. The ITF template is delivered in all folders but there is no simplified version provided out of the box: you need to set it up.
I strongly recommend you to be trained on Import/export before using this fonction. Mass changes in the system can easily lead to mass errors... Hence ensure a back-up is availabe in case of any issues and test the update process on the SEED folder before using it on the live folder.