My Xbox game wont update.

You could try clearing your cache, which would remove all updates, needing them to be redownloaded. Could fix the problem, but they again, could make it worse! support.xbox.com/.../clear-system-cache

Try deleting all related data for said game then try again.

have you checked your HD for corrupt files,also try clearing cache as well

Hi and thanks for reaching Microsoft Xbox gamer's community. My name is Josef.
I'm an Independent Advisor and a Microsoft Xbox user like you.

Hope you're doing great today despite this Saul

May I know if you tried power cycling your console?
1. Turn off your console by pressing the Xbox button on the front of the console for about 10 seconds, until it shuts down completely.
2. Unplug the console's power cable from the console and from the power outlet. Wait 5 minutes.
Important: Be sure to wait at least 5 minutes. This step resets the power supply.
3. Plug the console power cable back in.
4. Press the Xbox button on your console to turn it on.

Also disconnect your router from power for about 3 minutes. Then connect it again an when everything is working, try again

If yes, please try it again. But this time disconnect all your USB accessories before that and leave them disconnected
Once the console is turned on, try updating the game in that way

If maybe the issue persist, tell me:
What Xbox console do you have?
Are you using an insider build / beta OS on it?
If you try downloading a different game the same happens or is this happening only with Forza?

what game/s? how often? is your HDD full? can you try a different storage method (like a USB)? I need more info in order to help you