Via the module LokiCheckout_EditCart, the quantity of each cart item, typically shown in the sidebar, becomes modifiable. By default, this is presented with a text input field. But you can change this to a select box or something else easily.
The module already ships with a select.phtml to allow you to switch the input box to a dropdown:
<referenceBlock name="loki-checkout-edit-cart.sidebar.items.item.actions.input" template="LokiCheckout_EditCart::checkout/sidebar/items/item/actions/select.phtml"/>
When changing the quantity via this dropdown, the results are automatically submitted. Probably, this means that the Update button is no longer needed:
<referenceBlock name="loki-checkout-edit-cart.sidebar.items.item.actions.update" remove="true" />