Download the software zip file (PNXLSW_xxx.zip) and one of the bootfiles zip files. Both sets of files work together, so they must be from the same version.
Extract the software zip file and copy (overwrite) the contents to /var/www on the Pixie-Net's SD card. This is done most easily when sharing this folder via SMB over the network, i.e. with the Pixie-Net powered up and connected to the network, type \\192.168.1.67 (or equivalent IP number for your Pixie-Net) into the address bar of the Windows Explorer. You should see one shared folder (PNvarwww) which is the shared name of /var/www on the Pixie Net . Windows will ask for login credentials if you open this folder.
Extract the bootfiles zip file and copy (overwrite) the contents to the FAT partition of the Pixie-Net's SD card. This is done most easily by inserting the SD card into a Windows PC. Windows will detect it as a new removable drive and offer to show you the contents. This new drive is the FAT partition of the SD card. (Do not let Windows "scan and fix" the drive.)
You can also insert the SD card into a Linux computer which gives you direct access to the "boot" partition of the SD card.