In the Main window, make sure that List Editors are showing.
Click on the Tempo tab.
Click on the local [Options] menu and select [Tempo Operations]
You may also have or set a key command for [Open Tempo Operations].
In the Tempo Operations window, set your start and end points as well as your start and end tempos. You can play with the curve and density as well - I find that setting the density to 1/4 notes usually sounds natural.
You'll see a preview in the small graph.
Hit [Apply] - done.
You can of course manually enter tempo statements until you create the kind of rit you'd like.
Of course you can also just draw in tempo changes in the Global Tracks area, but the method above gives you a good controlled ritard, which you can also tweak later by simply adjusting the tempo values in the Tempo list.