If data exists replace

[bluns]

Hi all

I want my data to replace what is there if data exists instead of creating a new row.

I tried this

replace into contactdetails
(contactemail, emailsubject, dealershipdetails)
values
("Your Email Address", "Enquiry",
"Your Name, Address, Contact Numbers etc");

But this creates a new row if the data exists 🙁

Is there a command I am missing? Or will i have to check if data exists and if so delete it, and then command another insert query?

Thankyou!

[xblue]

from the manual: "REPLACE works exactly like INSERT, except that if an old record in the table has the same value as a new record for a PRIMARY KEY or a UNIQUE index" - which of the fields is mysql supposed to use in order to tell that a record set exists? is that one marked as unique?