It's very simple. Luckily the Trusted certificate store is available via FTP as cert/trusted directory
brano@droopy ~ $ ftp 192.168.10.1
Connected to 192.168.10.1.
220 FTP Server (ZyWALL USG 200) [::ffff:192.168.10.1]
Name (192.168.10.1:brano): admin
331 Password required for admin
Password:
230 User admin logged in
Remote system type is UNIX.
Using binary mode to transfer files.
ftp> cd cert/trusted
250 CWD command successful
ftp> dir
200 PORT command successful
150 Opening ASCII mode data connection for file list
-rw-r--r-- 1 root root 1553 Jan 20 02:48 bad_cert.crt
226-Transfer complete
226 Quotas off
ftp> del bad_cert.crt
250 DELE command successful
ftp> dir
200 PORT command successful
150 Opening ASCII mode data connection for file list
226-Transfer complete
226 Quotas off
ftp>
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