Use the JSON Editor

You can edit a schema directly as JSON in the TerminusCMS user interface using the JSON view

Create a schema as JSON

Now click on the pink bubbles on the left panel. This takes you to the schema builder page. Select JSON view from the tab and you'll see your entire schema as JSON.
If you click on the Edit button in the upper right-hand corner, you'll be able to directly edit the schema.
If you have no data in your database, it should be possible to freely edit the schema. However, if you have data, then you may not be able to make arbitrary edits. The schema editor will warn you upon submission if some restrictions are violated.
Essentially it should always be possible to weaken the schema safely through the interface. However, other changes will require schema migration which is coming soon.