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/master.cf 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.