How do I set a column value to NULL in SQL Server Management Studio?

I think @Zack properly answered the question but just to cover all the bases:

Update myTable set MyColumn = NULL

This would set the entire column to null as the Question Title asks.

To set a specific row on a specific column to null use:

Update myTable set MyColumn = NULL where Field = Condition.

This would set a specific cell to null as the inner question asks.

If you've opened a table and you want to clear an existing value to NULL, click on the value, and press Ctrl+0.

If you are using the table interface you can type in NULL (all caps)

otherwise you can run an update statement where you could:

Update table set ColumnName = NULL where [Filter for record here]