hi Marina-a,
you can try :
......
UpdateCommand.CommandText = "UPDATE Settori SET CODSET = ?, DESSET = ?,CODREP = ? WHERE CODSET = ?"
UpdateCommand.Parameters.Add("@CODSET", OdbcType.VarChar, 2).Value = cdset
UpdateCommand.Parameters.Add("@DESSET", OdbcType.VarChar, 30).Value = dsset
UpdateCommand.Parameters.Add("@CODREP", OdbcType.Double).Value = Cdbl(cdrep)
UpdateCommand.Parameters.Add("@ID", OdbcType.varchar,2).Value = cdset
....
Use Double instead of Decimal, like C programming with MySql API !