You get the new firmware file for the phone you need to flash - for me, the newest file I could find for the 410 series is V5 R6.6.0. Then you put it on an ftp server someplace you have access to. For a while I was sending them home, but recently downloaded a free little program called Core FTP Server and put it on my office PC. I have an IP phone on my desk, so I ask the 4000 what OP address it is on the Basic 2 tab under station, and then plug the phone in question into that cable, and statically assign it that IP address, because it seems like my problem is that the phone refuses to cooperate with our DHCP server.
Then you go into the setup menu on the phone, either by hitting it's IP address using https:// on your PC or by using the menu, which is slower, and you login to the admin mode on the phone using 123456 for the password. I am not looking right at it, but in the admin menu you will find a section to update the firmware. In there you will need to put in the IP address or FQDN (name) of your ftp server, and the login, password, and I think directory path (I usually just use / ), and the name of the firmware file for the phone, which is usually some long-assian thing that I usually shorten to something like 410stdv5r660 dot whatever. Especially if you're using the phone menu and have to key all that in! There will be one other thing in there that says something like "what to do after reset" that usually is set to "nothing". You want to change that to "download and install". Then you save all that, let the phone reset, and with any luck it will tell you it's downloading the new software, installing it, done, reset the phone after install, etc. Do not interrupt the process once it starts installing or you can brick your phone. With any luck it will complete and start up on the new version. With even more luck when you do your phone config and when you connect it to the network it will fire up and live happily ever after.
It works for me for most problems - as long as I can get into setup on the phone so I can download the update.
Please note my instructions are a very rough estimation of the process from memory, but it should be obvious from what I wrote when you see the menu

)