Within every channel there is a Files tab. Every document in this tab could be edited.
When you click on a document you want to view or edit, you will initially be in the Reviewing stat. if you want to edit the document, click on the Edit button on the top.
This will open a tab in your browser so you can edit the document. You can not edit any document inside Teams, it has to be done via the web App.
When you are the Editing stat, you can see the Edit History for that document and you can restore the document to a previous version.
To do so, click on File at the top bar, then Info > Version history. There, you can see on the right all the previous versions, choose the one you want to restore, then, click on Restore at the top.
Please contact us if you have other questions via submitting a Ticket here: