Initally I installed SQL server 2008 R2 Evaluation version. Then we bought licence key for SQL server 2008 R2 Web and wanted to update my licence, but unfortunately I installed analysis service when I was installing evaluation version which is not there in Web version so now I have to un-install analysis service. I googled this thing but found no real help.
Click uninstall in the "Uninstall a Program" wizard for SQL Server 2008. Don't worry, it won't uninstall everything. It will launch a screen that will eventually let you choose what instance you want to modify the features for. Just uninstall the Analytics Services and click next. It will leave the Database Engine and all of your data intact.