How do I change postfix port from 25 to other port?

If you only want it running on port 1025 (and I’m not sure you do; I’d think you’d want it running on both 25 and 1025), then find the line in /etc/postfix/ that looks like this:

smtp      inet  n       -       n       -       -       smtpd

And change it to look like this:

1025      inet  n       -       n       -       -       smtpd

If you want it running on both ports, then add the second line after the first one rather than replacing it.