Dropping Foreign Key without recreating table

I wanted to change a foreign key to be deferrable (db version 7.2.1).
During table creation I didn't specify a constraint name for the foreign
key.
"\d" shows a trigger RI_ConstraintTrigger_17195, however when I'm trying
to "alter table mytable drop constraint RI_ConstraintTrigger_17195
restrict" I'm getting the error "constraint does not exist.
What am I doing wrong here? Or is there another simple solution to
switch foreign keys to be deferrable?

Regards

Egon