Ummm...if the column is not a text column of some kind (that is, if it is a number or datetime, say) then you can *NOT* update the column to be ''
A more general answer is to update the column to NULL.
UPDATE tablename SET columnname = NULL
But even that can fail if the column was specified as NOT NULL
So there's no 100% general answer. It depends on the type of column and on whether the column allows NULLs or not.
If you do
and show us what it shows you and tell us what column you wish to remove the data from, we can tell you what will (and won't) work.