modify the recipient address of an email on the file in the postfix queue

The best way (or the least intrusive way) would be to have an address rewrite for that recipient. From the Postfix Address Rewriting

/etc/postfix/main.cf:
    smtp_generic_maps = hash:/etc/postfix/generic

/etc/postfix/generic:
    [email protected]       [email protected]

Following this example, you could try:

cd /etc/postfix

Add to the generic file or create it with:

[email protected]        [email protected]

and build the associated map:

sudo postmap generic

Add to main.cf:

smtp_generic_maps = hash:/etc/postfix/generic

make Postfix load this configuration change:

sudo postfix reload

smtpd will use generic to rewrite the problematic address upon next scan of queue.