how does one replace a row in Postgresql? (ala MySQL REPLACE)

A common task is to check to see if a row with a certain primary 
key exists before inserting it. If the row exists, then you 
simply update/overwrite its contents with the new row data. 
Otherwise you insert it.

MySQL has a REPLACE command which does this.

I didn't see one in the Postgresql manual. How do you do this in 
Postgresql?