Modify Trigger in Postgresql

Yes. According to the documentation:
http://www.postgresql.org/docs/9.0/static/explicit-locking.html
Also, most PostgreSQL commands automatically acquire locks of appropriate modes to ensure that referenced tables are not dropped or modified in incompatible ways while the command executes. (For example, ALTER TABLE cannot safely be executed concurrently with other operations on the same table, so it obtains an exclusive lock on the table to enforce that.)

will it "pause" the old trigger while it is updating the function?

It should continue executing the old trigger functions when calls are in progress (depending on the isolation level, subsequent calls in the same transaction should use the old definition too; I'm not 100% sure the default level would do so, however), block new transactions that try to call the function while it's being updated, and execute the new function once it's replaced.

As far as i know, Postgres should execute all the REPLACE FUNCTION in one transaction (so the tables are locked and so the triggers being modify while it is updating, then next transactions locked will use the new FUNCTION not the old one. is it correct?

Best I'm aware the function associated to the trigger doesn't lock the table when it's updated.

Please take this with a grain of salt, though: the two above statements amount to what I'd intuitively expect mvcc to do, rather than knowing this area of Postgres' source code off the top of my head. (A few core contributors periodically come to SO, and might eventually chime in with a more precise answer.)

Note that this is relatively straightforward to test, that being said: open two psql sessions, open two transactions, and see what happens...