You're in luck; I didn't get enough sleep and am waiting for the caffeine from my coffee to kick in, and while that's happening, I've authored this answer:
You ask for help "how can i change port 25 into SSL certificate", but that goal doesn't make any sense. A "port" is a feature of networking rather like a door, while an SSL certificate is... like a certificate, meaning a document; you're asking to change a door into a document.
One can guess you mean you want the door - the port! - to be able to accept the certificate, but it's very obvious you don't yet have the background to know what you're doing. This MAY be able to push you in the right direction:
While I'm not an expert with RC, as someone who's a very experienced system administrator, I guess you mean "VPN" instead of "VPS" as I don't even know what VPS is, but a Virtual Private Network is a real thing and would almost make sense in this context.
But if a vendor sold you a VPN, they should explain to you how to use their product. However, I wouldn't expect this would be of their direct concern because you, as system administrator, have to configure Round Cube (and any other software you run, such as your Mail Transport Agent - MTA - such as postfix and IMAP server such as Dovecot) to work in this environment. And generally speaking non-standard port assignments aren't going to "play well" with other people's email systems.
That said, the ports involved are:
25 is for "SMTP", which is normal, inbound, unencrypted email
587 is for "mail message submission", similar to port 25, but used for "outgoing mail"
993 is for IMAPS - that is IMAP over SSL
995 is for POPS - POP protocol over SSL
465 has (at least) dual use but started out as used for "SMTPS", meaning SMTP using SSL (TLS) It's officially deprecated for that use but is still often used for this.
2525 is an unofficial port that many sites use for SMTP submission (outbound) as an alternative for 587.
There's a lot of help on the internet, such as this page - it's not the most accurate, such as using the word "web" when they clearly mean "internet", but is intended for beginners like you:
https://kinsta.com/blog/smtp-port/