I think you can just include a button in the form. Go to your cart.liquid, if all you see is {% section ‘cart-template’ %} then open cart-template.liquid in your sections folder.
Find the form, and look for your submit button. You can probably just ctrl + f for “submit” and find it. Just place this by it:
Then check and see if it works, I’m not really sure since all the themes I checked have this button, and this is what it appears to be. I believe the type just has to be “submit” and the name just has to be “update”. Then just style the button however you want.
You can send me a DM with your store URL and I can request access to your themes and take a look for you if you don’t mind. I’ll duplicate your live theme and make the changes on the duplicate.