The latest version (8.4.0.0241) of CM does not allow to change the UNIQUE constraint of a column through GUI Edit Column window.
1. Right click on a table and choose Edit Table.
2. Choose a column which has UNIQUE constraint.
3. Click Edit... button to edit the column.
4. The UNIQUE constraint field is disabled.
To change the UNIQUE constraint one needs to:
1. Either execute SQL in Query Editor/CSQL.
2. Remove the UNIQUE index from the FK/Indexes tab in Edit Table window.
All these options are inconvenient for users. They have to be able to add/remove any constraint right from the GUI editor.