Here are the steps:
1) Boot into Fedora
2) Install GParted if it's not already installed
3) Run GParted, and write down (on paper, or in gedit, kwrite, or wherever) what partitions exist and where on the drive that you are interested in.
If you don't have any volume labels, it might be a bit harder to discern which is which, but mounting and rummaging around those partitions should give you the answer. (To help you start, look at the partition formats: NTFS = Windows, ext2/3/4 = Linux, swap is obvious.) Be very careful to note any purely-data partitions; you don't want to destroy your /home partition if you have one, for example!
4) Copy any data from the Ubuntu partition you want to save back to your Fedora partition (in Nautilus, Dolphin, the command-line, or however else you like.)
5) In Fedora, reinstall the boot-loader by going to the command-line (aka terminal), and typing:
su -c '/sbin/grub-install /dev/sda'
Give it your root password when it asks for it.
IMPORTANT NOTE: Replace /dev/sda with whatever drive your OS partitions are on if needs be.
6) Unmount the Ubuntu partition, and then use GParted to delete the Ubuntu partition. You may now expand the Fedora partition to take up the space that Ubuntu used to, assuming that the Fedora partition is immediately preceding the Ubuntu one.
That should be it! There are a couple of caveats though:
Since you installed Ubuntu after Fedora, it shouldn't be an issue, but if you had to delete a partition that wasn't the last one, you'll have to update GRUB with your new partition numbers. To do you, you need to edit /boot/grub/grub.conf root. The specifics of doing so are beyond the scope of this tutorial, but if you need to do so, feel free to ask how.
This is all assuming that you have a normal partition layout; if you installed Fedora into a LVM, as it might if you let it do the partitioning for you when you installed it, it might be a bit more complicated.