concatenate 2 columns into 1 and update it in the table using sql query

I have 3 columns in a table paxname,fname,lname...now i want to concate fname and lname and update that in paxname...so i ran the following query but it does not work...can anyone please help me with this

update pax set paxname=lname || " " || fname

I tried with CONCAT function but it didnt work either.