If it is in STEAM, then press the UPDATE button and it will update your core game to the latest version. If it is in the game, it is some update for a car or track you already have. Check the Packages screen to see if you have any un-installed content or exit rF2 AND Steam and restart both.
Hav those too and cannot get them installed. Tried by content manager and ingame - now have 15 pending.....
I had three, they were all stuff I had in my packages folder and impossible to install.... the reason for me was that they were all personal updates to tracks that no longer worked because the base track had been updated.... deleting them out of my Packages folder removed them from the pending screen. When you're in the pending install screen check to see whether they are Local or Workshop, if local check your packages folders for the offending items and delete if necessary.
I have 61 pending updates in my local folder and they aren't personal updates. I think it's something in the latest RC update.
The latest RC I can not install my own Bathurst track to test in game. Also other track will not install. Just get the yellow tick. Also on exit, the game hangs for ages. I use Task Manager to close the game. This is only the latest patch/RC I have gone back to V1131 and all is good.
I had some of my own builds for the Formula E cars that wouldn't install after the RC. I removed them from the install folder and rebuild them in the mas2 editor. Afterwards they were fine.