The SQL command will be as follows:
ALTER TABLE <tablename> ADD CONSTRAINT
<constraintname> UNIQUE NONCLUSTERED
Follow these steps, if you want to use the Database Diagram:
Step 1: Right-Click on the table and then select 'Indexes/Keys'
Step 2: Click the Add button to add the new index
Step 3: Enter the needful information in the Properties which you'll find on the right-hand side: the columns that you want to set it as Unique to Yes and give an appropriate name.