This will depend on the distribution you are using.
If you use debian and 'synaptic' as package manager, all you have do do is:
1. search for package named "ibqt3-mt"
2. open the "versions" tab
3. see, that there currently is
- 3:18.104.22.168b-1 (unstable)
- 3:22.214.171.124-9 (testing)
- 3:126.96.36.199-4etch1 (stable)
4. change preferences/settings/distribution to "(*) prefer: <testing>" (guessed, as I have a German KDE, that reads: "Einstellungen/Einstellungen/Distribution: (*) Version bevorzugen von: <testing>")
5. select "ibqt3-mt" and press Ctrl&E to explicitely select a version; choose "testing" (- or just perform a deinstallation -)
6. reinstall the package (this performs the downgrade)
If you really want to download the deb-file,
go to http://www.debian.org/distrib/packages
and search for "ibqt3-mt" (in "all" distributions). At the moment, you will come to http://packages.debian.org/lenny/libqt3-mt
, where you can choose a mirror to download the desired file. After that, install it by 'dpkg -i ibqt3-mt.deb' (or open it from konqueror with kpackage, which will assist you graphically to do just that).