How big is it? Even a whole windows ISO is only about 3GB so you shouldn't have anything huge in there.
You can clear everything in C:\Windows\SoftwareDistribution\Download\ (you lose the ability to roll back a patch but that isn't important as Windows deletes things eventually anyway.).
It shouldn't be locked unless you are actually half way through installing an upgrade. If it is still locked in say 30 minutes and you want to delete is I would try these in this order:
1. Boot into safe mode and try to delete it.
2. Open a command prompt at boot and enter the following (be careful - just enter the bit in red exactly as it is).
There are various third party tools (like unlocker) to do the same but I couldn't find a link that wasn't full of malware so I'd not bother. It is safer to use Windows tools and easy enough as long as you are careful and don't delete the wrong thing.
Microsoft Windows [Version 10.0.10586]
C:\>pushd "c:\Windows\SoftwareDistribution\Download" && (rd /s /q "c:\Windows\SoftwareDistribution\Download" 2>nul & popd)