Read carefully, because if you do something wrong, the router will be bricked, and the only way to unblock it will be a serial cable!!

OFW to FreshTomato:
 1) download both files for N60
 2) restore default settings!! (default ip will be 192.168.0.1)
 3) upgrade router using TendaN60-init image
 4) wait ~ 4-5 mins
 5) default ip will change to 192.168.1.1
 6) log into FreshTomato and erase NVRAM
 7) upgrade router using TendaN60 image
 8) done

Return from FreshTomato to OFW
 1) erase NVRAM using FreshTomato GUI
 2) upgrade router using TendaN60-init image!!
 3) erase NVRAM one more time
 4) upgrade router using Tenda's firmware
 5) wait ~4-5 mins
 6) default ip will change to 192.168.0.1
 7) log into Tenda GUI and restore default settings
 8) done