Since you're on Windows and are probably starting PostgreSQL as a Windows service, you'll need to edit
postgresql.conf (inside your data directory) to set the new port there, then restart the postgresql service using the Services control panel or (as an Administrator) the
net service command.
You can use a programmatic text editing tool to change
postgresql.conf if you need to automate this. On a Linux/unix box you'd use a tool like
awk, but on Windows I imagine you'll be using VBS, powershell, or an add-on tool like Perl.
You'll need to change the connection settings in tools like
PgAdmin-III to use the new port.
If you were starting PostgreSQL manually using
pg_ctl you could set the
PGPORT environment variable or pass a config option on the command line. This won't work when you're launching it as a Windows service.