Hi Frank,
You are right, you must use an UPDATE statement to change the value of a column in a particular row.
The syntax is (taken from MySql.com):
UPDATE [LOW_PRIORITY] [IGNORE] tbl_name SET col_name1=expr1,col_name2=expr2,...
[WHERE where_definition] [LIMIT #]
In your example, this would be:
UPDATE Ggerwyt SET merURL='test.com'
WHERE GgerID=12
Hope that helps!
Clarissa