I would use SSMS Table Designer, make the changes there but do NOT press the [SAVE] button. Instead press the "Generate Script" button, save or Copy&Paste the Script somewhere safe and then ABANDON the changes (in the SSMS Designer).
SSMS generally makes good scripts for DDL changes, and if nothing else it will show you a decent, methodical, script for the change you wanted to make, which has some feedback/learning benefits too. No "human error" in the scripts that SSMS generates either
Ideally your CLUSTERED INDEX should be unique. Obviously if the keys in it are not unique then ... No! ... but if they are (or SHOULD BE!!) UNIQUE then definitely include that attribute. Otherwise SQL will add an extra (internal) KEY column to the clustered index to MAKE each entry unique AND those extra bytes will also be included in every row of every non-clustered index on that table too - so potentially "quite a lot of bloat" !!