If you need to change the TTLs on all domains in Plesk at once, you can massage the database with the following command:

mysql> UPDATE `dns_zone` SET `ttl` = '300', `ttl_unit` = '60' WHERE `id` >1;

Substitute the TTL values for what you need. Since the flat files are still used by named to provide DNS resolution, those will need to be updated as well. This command will do the trick:

mysql -Ns -uadmin -p`cat /etc/psa/.psa.shadow` -D psa -e 'select name from domains' | awk '{print "/usr/local/psa/admin/sbin/dnsmng update " $1 }' | sh