In short: Both items are safe to cleanup/delete right away, assuming as your computer is working properly and you don’t plan on uninstalling any updates currently installed or need to revert back to an older build/version of Windows.
More information:
1. Previous Windows Installation and Windows Update Cleanup simply are old system files that are kept by PC in case you wish to revert to old build/version of Windows or uninstall updates (respectively).
2. Windows 10 gives ~10 days or so of time to revert back to old build after doing an upgrade; if nothing is done and you let that window of time expire, then Windows would delete the Previous Windows Installation files and not allow you to go backwards anymore to save you disk space. (I am not sure if Windows 10 allows you to go backwards to more than the build that existed right before your upgrade was done)
3. I believe Windows doesn't delete Windows Update Cleanup automatically though (not 100% sure with that) but you manually ticking the checkbox and cleaning those files if you wish is possible.
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If paranoid, you can always do a disk image backup in case anything goes haywire but if your computer runs fine and you can do all you need to do on your pc without issues as is, then deleting those will just give you back disk drive space.
Also those files as your screenshot shows, can take up significant space on your computer. (I also see screenshot indicates an whopping 46.4GB you can cleanup, which is for most users not a trivial amount of space!!)
