You can quickly update the medium of multiple artworks at the same time with the bulk edit tool. On the artwork homepage, select the artworks you would like to update by clicking the grey box on the left of the list, turning it blue. Don't forget you can filter, sort and search to find the artworks you are looking for.
At the bottom of the page select View Selected to see your list. Then click More Options to open the menu. Now select Medium from the Bulk Edit options.
In the pop up that appears, click into the Medium field to open a drop down list of options to choose from. Select the medium you wish to use.
Instead, you could create a new medium. To do this, click Create User Entry and type in your new medium into the field. Press enter on your keyboard to save. You will see the New Value column updated with your new medium.
Now press Submit to update your artworks.