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.