Most likely the last firmware for these cameras
edit 24Feb20 In fact it's the 5.4.41 that's the last version.
Privilege Escalating Notice by Hikvision that firmware IPC_R0_EN_STD_5.4.41_170312 fixes the 'backdoor vulnerability' that is present in almost all older versions of the camera firmware.
If your camera firmware is older than 5.1 you will need to update in steps. Start with 5.1, then go to 5.2, then 5.3, etc.