The best way to reinstall iRacing is to first back up your Cars and Tracks folders to another location.  

Go to wherever you installed iRacing (by default this is C:\Program Files (x86)\iRacing), right click the CARS folder, choose Copy and then paste it outside of the iRacing folder (for example, to your Desktop). 

Do the same for the TRACKS folder.

Then 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.

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.

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.