ln ttyS1 ttyS0
Alternately you could do this:
mv ttyS1 ttyS0
Or if you really want to put the smack down:
mknod ttyS0 c 4 65
chown root.dialout ttyS0
chmod 660 ttyS0
You could, of course, set the mode and ownership however you want . . . no idea what distro you are using . . . my example shows the proper Slackware way.
Be sure your circuits are all properly grounded! Of course, your machine has four UARTS . . . so hey . . . you could make nodes of type c all the way up to minor 67!