Error Upgrading Archer c60 v3 OEM firmware

I have recently reverted back from openwrt by flashing a modified firmware which i found on this forum but this modified version is very old and i want to upgrade the version , so when i am upgrading the version it says some error which i forgot something like bad format file. after searching through this forum i found that i have to modify the version number then recalculate the checksum which i cannot do and i have 0 idea about this, can someone please help me