As root, you can unmount the CDROM with /sbin/umount /cdrom, take it out of the drive, insert another one, and mount it with /sbin/mount_cd9660 /dev/cd0a /cdrom assuming cd0a is the device name for your CDROM drive. The most recent versions of FreeBSD let you mount the CDROM with just /sbin/mount /cdrom.

Using the live filesystem—the second of FreeBSD's CDROM disks—is useful if you have got limited space. What is on the live filesystem varies from release to release. You might try playing games from the CDROM. This involves using lndir, which gets installed with the X Window System, to tell the program(s) where to find the necessary files, because they are in the /cdrom file system instead of in /usr and its subdirectories, which is where they are expected to be. Read man lndir.