To update an already existing assembly, click My Assemblies (located in the Menu Bar). You will be redirected a list of assemblies that you have previously created and saved.
Click the Name of the assembly you want to update to open a new tab of this assembly in the Anekonnect editor. You can now procceed to edits or updates to the assembly. Then click the Update button located on the right-hand side of your drawing (where the Save button is typically found).