1)Stop OpManager service
2)Go to OpManager command prompt with Run as administrator Option
3)Change directory to OpManager/bin and execute " opmanagerservice.bat -r "
Ex: C:\ManageEngine\OpManager\bin>OpManagerService.bat -r
(The above command will remove the OpManager service )
4)To reinstall the service , execute command "OpManagerService.bat -i"
Ex: C:\ManageEngine\OpManager\bin>OpManagerService.bat -i