1.In Add/Remove programs
2.Click the 'Uninstall/Change' button for "SQL Server 2008 R2"
3.In the coming dialog, click 'Remove'
4.Click 'OK'
5.In the 'Select Instance' dialog,  select the appropriate instance of the SQL Server (!) and then click 'Next'
6.In the 'Select Features' dialog, please only select 'Reporting Services'
7.Then click 'Next' and 'Next' till the uninstalling process starting

Step 6

Step 7