from OpenWRT wiki http://wiki.openwrt.org/toh/tp-link/tl-wr710n

- connect to the device with FTDI
- type 'tpl' at boot time to get U-boot console
- in console type
    setenv ipaddr 192.168.1.1
    setenv serverip 192.168.1.2
    tftpboot 0x81000000 <firmware-file.bin>
    erase 0x9f020000 +0x7C0000
    cp.b 0x81000000 0x9f020000 0x7C0000
    bootm 9f020000

the same goes for restoring to the original fw, it is available here
wr710vn1-130419-orig.bin