Please try stopping your Helper Service that is running in the background.
To do this open your Windows Task Manager and find the process running called "iRacing.com Helper Service". Right click and choose End Task.
Then try and complete your update within Steam.
If that does not help. You can try the following:
1. Close out Steam, the iRacing UI, and the members site completely. Log out of each application and make sure that it's not running in the background.
2. Clear your browser of all cache and cookies.
3. Then uninstall and reinstall the service. Possibly on a different drive if you have another that has more space available.