Insert into record where 2 fields must match

[singingsands]

I want to insert (not replace) data into a record that already exists. The user would not know the ID of the record so I have to use a concatenation of 2 fields to get the correct record. This is the sort of thing I want but I know it isn't right.

INSERT INTO table ('field5') values('value1') WHERE ('field1') == 'value2' && ('field2')=='value3';

What's the right way to do this?

[Barand]

Inser queries cannot have a WHERE clause. You need an UPDATE query

[Phi11W]

18 hours ago, singingsands said:

I want to insert (not replace) data into a record that already exists. 

You INSERT new records.  

You UPDATE existing records.  When you do this, you replace the values that were in that record before you updated it. 
There is no way around this - it's just how databases work. 

What you're attempting to do is an UPDATE: 

UPDATE `table` 
SET field5 = 'value1' 
WHERE field1 = 'value2' 
  AND field2 = 'value3'

Regards, 
   Phill W.