Let's say you have a Closed Sales Order that you want to revert to Invoiced.
In order to be able to reopen closed documents, you should have -firstly- enabled the corresponding permissions. Navigate to your account's permissions:
Then, click Edit Users Permissions next to the username you want to apply this for and in the Permissions screen, click More next to Documents (Sales Quotes and Orders ):
After that, tick the option on the top of the page so that your user can re-open closed documents.
Now, navigate to the Sales Order you want to re-open.
Click More and see the following menu:
Re-Open Document option reverts your Sales Order from Closed.
If you want to further revert the Sales Order from Invoiced to Approve and re-edit the quantities and the prices of a product, check this article .