This article assumes that you have created a schema. if you haven't created one you should do that first.
Start by clicking the Edit Metafields button in the top navigation menu.
Select the object type you want from the dropdown and click the Show Results button.
Below the Show Results button you'll be presented with the bulk update section. now enter the fields that you want to update.
Once you've entered the new values for the fields in the bulk update section you need to select the objects you want to update.
Then click Select All to select all of the products.
Finally click Update Selected and your objects will begin updating.
Note: This will take some time and you will be notified when each page of results has finished updating