Hi Ravi,
where did you get the new version from? To be sure that you have proper version, please make sure you get the file from your management server, from the following location:
C:\Program Files\Microsoft System Center\Operations Manager\Server\AgentManagement\amd64
The file should be called MOMagent.msi.
Now to the second part - the upgrade. The main problem that you need to check for is why you are unable to install the agent. Exactly because there is a problem with the uninstall, you are also not able to upgrade. The upgrade process basically uninstalls the old version and instals the new one afterwards.
Has your agent been installed with SCCM? If so, please look at this:
or you can try a force remove as per the guide of AlexZhu here:
Please let me know how it looks. Thanks.
Regards,
Stoyan