Remove all Null values from Sql table at once

I am very new to Teradata and SQL environment. I am trying to remove all the null values from sql table in teradata using ISNULL(COLUMNNAME,0). But it gives an error that "the data type does not match the defined data type name". Also, would like to know if there is any way of removing all NULL at once rather than column wise writing the above query again and again.