To add weights to your products please go to inventory-> inventory & listings
From here you can select and edit any product in your inventory
Next, please select the shipping tab.
From here you can select the blue link, 'bulk update'
From this page, you can download a sample file, or create your own CSV file using the available headers in this list.
The headers you can use to update the weights and dimensions of your products are as follows:
SKU - this must be used to assign the correct weight and dimensions to the correct product
dimension_unit -This is the dimension_unit e.g cm for centimeters, inch for inches, m for meters
Please find the attached sample file used in the video below.