Changing a trigger function

I have found a nasty bug in one of the trigger function I did put into
production a couple of days ago. I have a fix for it but wonder if I
can put it into the production database with users connected on it.

What would happen if someone sends an update firing that trigger
whilst I am in the middle of updating it via a create or update ? Do
I need to wait for this evening or can I safely runs the update of the
function ?

I know that several other databases would reject the update (Sybase,
Firebird for example) but I am too new to PostgresSQL to know what
would happen in such a situation

Didier