Take both drives out of the NAS, put one safely to one side, attach the other to your PC either directly or using a USB-SATA cable/enclosure. If you are using Windows, install EXT2FSD (
http://www.ext2fsd.com/) so that you can mount and read the data partition. Mount the partition as "read only" for safety, you will be asked during installation. If you are using Linux, you do not need a filesystem driver.
Install the new drives in the NAS and format as RAID1. Copy the data from your computer to the NAS. Do this over a wired Ethernet connection if possible for best performance and stability.
There are harder ways to do this using third party tools and the USB port on the 323.