Microsoft made some temporary adjustment to Office document behavior within Teams to accommodate an increase in usage recently. There will be an additional step in order to edit a document in Teams. This is a change from current behavior that allows you to go straight to edit mode when clicking on a document in Teams.

Now, upon clicking on the document, you will first see the document in view mode.  In order to edit the document, you need to click on the options in the top right of the white bar above the document content and select "Open in Browser" or (for users with Office for Windows installed on their device) "Open in Desktop App". This will open the document in Office in a separate window from Teams and allow you to make changes.