While not easy, by my standards at least, this is the way to update the ant plugin. At least for RAD 8.5, I'm sure eclipse is similar.
Go to Help -> About RAD -> Intallation details -> Plugins ... locate the org.apache.ant plugin version you are using. Note the Provider ... in this case for me it was Eclipse Orbit.
Next you need the repository URL for Orbit, googling got me this page.
http://wiki.eclipse.org/Orbit/FAQ
Which describes how to locate the repository.
Find the Orbit build drop that you need here:
http://download.eclipse.org/tools/orbit/downloads/
Add 'repository' to the URL of the drop. For example, for the drop in
http://download.eclipse.org/tools/orbit/downloads/drops/S20101204061544/
the URL that you would use is:
http://download.eclipse.org/tools/orbit/downloads/drops/S20101204061544/repository/
I personally went with
http://download.eclipse.org/tools/orbit/downloads/drops/R20130827064939/repository
Next, if you are behind a proxy, make sure that your proxy config is correct and the IDE can connect to the internet.
Then go to Help -> Install New Software
Next to the Work With dropdown hit the Add button and enter the repository information.
Select the apache bundle from the list and complete the steps of the wizard to perform the update.
Now that the repository is added to the list, for future updates you can use the Help -> Check for Updates menu item.
Good Luck, took me a day to figure that out and work through it.