Try temporarily disabling your antivirus application and if you have a third-party firewall installed, enable the Windows firewall temporarily but disable the third-party one.
This has solved the same problem for many other people according to what I've read elsewhere on other forums. Seems to be a common problem with Net Framework updates.
If this solves the problem and the update installs okay, you can re-enable your antivirus and third-party firewall, but remember to disable the Windows firewall as having two firewalls running together is not advisable.
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