Put a space between "1" and "Where".
"Update" + TableName + "SET" + DelField + "=1 Where" + IDField + "='" + ID + "'";
or you want to set DelField value by variable then
"Update" + TableName + "SET" + DelField + "="+DelFieldValue+" Where" + IDField + "='" + ID + "'";
I also recommend you to use SQLParameters for these type operations.