1) backup any data you don't want to lose to either, space on a computer hard disk, or a USB disk (note the DNS-320L only has a USB 2 interface.
1a) if you have been backing up your data make sure the backup is up to date and readable.
2) check that the firmware will accept the larger hard disks (you need two for RAID 1). If it doesn't, update it.
3) shut down the DNS-320L and unplug the power connector.
4) remove the disks (mark which slot they come from just in case)
5) insert new disks and power up the box.
6) use a browser to access the NAS web interface (example enter http://192.168.0.32 as the browser address - use the IP that you gave the NAS at setup) don't forget your password.
7) click on the 'Management' tab and then 'Disk Management' on the left and then 'Hard Drive Configuration' and finally the 'Set RAID Type and Re-Format' button.
after the disk array is formatted restore your data from your backup.