Hi there,
By default, if you click the “Remove item” / “X” icon to remove an item from the cart, then it should include an “Undo” link like this:
Link to image: https://d.pr/i/vkyvGx
However, when decreasing the product quantity and then hitting the “Update Cart” button, then it doesn’t have that Undo button since 1) the customer might be adjusting several product quantities, not just removing a single item, and 2) there are more steps involved to remove the item that way, so it is less likely that they would accidentally do it.
If you are not seeing the “Undo” link like in my image above, then it’s possible your theme might be changing that — please try switching to the default Storefront theme to see if that makes a difference.
If you are still having trouble, please share a copy of your site’s System Status. You can find it via WooCommerce > Status. Select “Get system report” and then “Copy for support”. Once you’ve done that, you can paste it here in your reply. Please note that since this is a public forum, you may want to remove any sensitive information that you don’t want shared publicly.