A quote can be edited by the shop owner afterwards, as long as it hasn't been accepted yet. This way, you don't have to create a new quote for the customer if changes need to be made to the line items and quantities. The time limit on the quote remains unchanged.
Open the relevant quote in the backend.
In the quote dialog, click the Edit cart button. This automatically redirects you to the cart overview of the shop frontend in the customer's context (i.e. you act in the frontend as if the customer had logged in themselves).
Now you can remove products, change the quantity of the products included, or add further products as you like.
In the cart overview, use the Update quote button to save the new cart.
A new quote document is automatically created from the new cart of the quote, which can be found in the Document history tab as usual.