is it possible to upgrade from 18 to 20 via USB flash drive

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Is not possible

The new version of installation manager fedora "anaconda", does not update the old versions. To update you must use online resources like fedup

You can use the command #yum --releasever=20 distro-sync

using Fedup #fedup --network 20

Using Fedup offline download image fedora #fedup-cli --iso /home/<user>/fedora-20.iso Once the preparations have completed, check the fedupdebug.log file if any errors show up in the output from fedup-cli.

If you want to upgrade to fedora 20 Make use of this in the terminal window.

rpm --import https://fedoraproject.org/static/246110C1.txt
yum update yum
yum clean all
yum --releasever=20 distro-sync