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]

Ctrl+0 or empty the value and hit enter.

Just as a little extension to Jeff Martin's and Zack Peterson's solution.

If you still want to set several values from the columns to null you can simply set the query to

UPDATE myTable SET MyColumn1 = NULL, MyColumn2 = NULL, MyColumn3 = NULL, ...

CTRL+0 doesn't seem to work when connected to an Azure DB.

However, to create an empty string, you can always just hit 'anykey then delete' inside a cell.