That firmware is pretty old.
The current release is 6.3.3, but if you aren’t doing Anything but managing multiple WAN links (no AP controller, vpn tunnels, or VLans) - jump to 7.0.
You can find a link in the forums - just search for firmware 7.0.
You will most likely need to download the firmware to a computer and do a manual upload. Just make sure you download a copy of the config first. Always backup your config!
I don’t work for Peplink and there may be some issue with jumping that many firmware versions - please use this advice at your own discretion. The employees that moderate the forums will help you much better than I can. I guess this advice is about worth what you paid for it -