To be more specific I would like to add an "update cart" button so that the visitor can adjust the quantity of multiple cart items without having to click on "update" for each cart item.
Not at the moment, you can submit a feature request for the feature here: https://github.com/craftcms/commerce/issues
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Thank you for taking the time to reply Luke. Hopefully, that's near the top of the list for upcoming updates. – Panos Spiliotis May 26 '18 at 9:17
If you're still using Commerce V1 - MultiAdd can do this - https://github.com/verbb/multi-add
