Upgrading fedora with ISO

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The recommended method is to use fedup. This method is safer than using yum alone.

I'm not sure if i understood your question.

If you have Fedora 19 already installed, you can simply upgrade to Fedora 20 using YUM:

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

This will not touch your partitions, it will simply upgrade your software, bear in mind that Fedora 20 is not stable yet, so be prepared for a few bumps on the road.

Please read more at https://fedoraproject.org/wiki/Upgrading_Fedora_using_yum

When an internet connection isn't avaiable you can download a CD or DVD with Fedora, and then on your computer set the repository for your CDROM:

1 - Download and burn CD or DVD with Fedora 20

2 - Put the CD on your computer and see where it's mounted

3 - Create a new repository file on /etc/yum.repos.d

sudo gedit /etc/yum.repos.d/cdrom.repo

[cdrom]
name=Local CD Repo
baseurl=file:///mnt/cdrom

Change the /mnt/cdrom to your system directory where the CDROM is mounted.

On all other repositories change the enabled tag to 0

enabled=0

Save the file.

Now you can use yum normally, including like i said on step 1.