The best way to reinstall iRacing is to first remove it from your Windows Control Panel...Add/Remove Programs. 

Then delete the C:\Program Files (x86)\iRacing folder.

Empty your Recycle Bin.

Then reboot your computer.

After that, access and follow the download/install/update instructions.

You can find more details in this FAQ: 

Although your setups and replays should not be affected in this process, if you feel more comfortable, you can first back up your Documents/iRacing/Setups and Replay folders first.

To save time (and bandwidth), you can back up your C:\Program Files (x86))\iRacing\Cars and Tracks folders to another location.

Then after deleting and reinstalling the iRacing program, before doing the download/updates - replace the new Cars and Tracks folders with the original ones. Then go back to the iRacing Members site and click Refresh. You may still need to do a small update but it will be much quicker.