The interface between the Pi and the AVR is serial, sometimes there are errors for reasons I can not nail down. The AVR sends thousands of lines of data per hour so if your error number isn't going up every time you refresh the page you shouldn't worry about it.
I need to put some better documentation in the wiki on the software upgrade procedure, but it's pretty easy
Upgrade OpenWrt firmware (Pi/SD Card): Go to System -> Flash Firmware. Paste the url of the .gz image in the Image URL box and press buttan.
Upgrade AVR firmware: Go to LinkMeter -> AVR Firmware. Select "from online repository". Press buttan.
The larger question to be answered is what you want to flash and where to get it, which is a little disorganized at this time.