How to update Windows 10 when you're low on free space

[Windows Central]

Whether you use Windows 10 on a tablet or laptop with limited storage, or you have managed to fill up your hard drive, the lack of available space doesn't mean you can't upgrade to a newer version. The truth is, you can install new releases of Windows 10 on virtually any drive. You just need a pair of USB flash drives.
Although there is a feature known as "reserved storage" to set aside a percentage of available storage to avoid problems during updates, it's not available on every installation, which means that you'll still need to use this workaround in many cases to proceed with the upgrade.

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