Here vxdiagshop.com provides DoIP update tool free download:
Free download vxdiag update_doIP.exe
The Update tool is used when you failed to update VXDIAG Benz C6 firmware to v1.5.3 or above.
That is because your VXDIAG C6 firmware is too old. You need to download the update tool to update it to version 1.5.3 first.
Then auto update it to newer version (latest V1.5.4 till May 2019) in VX Manager.