Once your tables contain some data, you have the option of changing existing records. The most frequent reason for doing this is if information has been entered incorrectly. Or, in the case of user information, if data gets changed (such as a last name or email address) and that needs to be reflected in the database.
The syntax for updating columns is
UPDATE tablename SET column=value
You can adjust multiple columns at one time, separating each from the next by a comma.
UPDATE tablename SET column1='value', column2='value2'...
Normally you will want to use a WHERE clause to specify what rows to affect; otherwise, the change would be applied to every row.
UPDATE tablename SET column1='value' WHERE column2='value2
Updates, along with deletions, are one of the most important reasons to use a primary key. This number, which should never change, can be a reference point in WHERE clauses, even if every other field needs to be altered.
To update records:
UPDATE clients SET client_phone = '(800) 123-4567' WHERE client_id = 2 LIMIT 1
With this query it's unlikely or impossible that more than one record would be updated, but the extra insurance is nice.