Check the status of the order.
Changes can only be made with status Queued and Backorder.
Block the order so you have time to make the changes
Insert a reason in the pop up (e.g. modify article)
Now select Edit to be able to make modifications to the articles
Scroll down to Lines
You can select different options:
Delete line: this will remove the complete line/article, regardless of the number of articles
Edit: a new popup will appear where you can change the quantity. Modify the quantity and click on Save
When done with the modifications, don't forget to unblock the order
Please note that the order in Monta must match the order in Magento.
If articles are removed/changed/added, the order in Magento must be modified too!
Please see Orders - Changing an order for more information