Please follow the instruction below, or you may not upgrade successfully.

If your current Firmware Version is 5.0.0 Bulild 120522 Rel.23978, please choose TD-W8951_V5_140226 to upgrade.
If your current Firmware Version is 5.0.0 Build 131104 Rel.09041, please choose TD-W8951_V5_140306 to upgrade.  

Both of the firmwares can't be downgraded.