You can simply drag and drop documents into an existing collection.
To move a document to a new, go to your documents console and find the document you with to move. In this example we will move "Tutorial" to "Invoices":
Select the more_vert menu icon. You will be presented with a list of options, select "Change Collection":
Start to enter the name of a collection and Simul will provide suggestions:
Select "Move", your document will now be in the new collection.