Go to View - Tempo. This will show a tempo map in bars and beats. Here are some options:

At the top of the screen there are 5 icons:

The arrow selects a region.
The pencil draws tempo according to the grid (below)
The line draws a smooth straight line
The eraser deletes tempo information as you select it and returns it to a value
The grid turns snap-to-grid on and off. By default, the draw tool will draw tempo per measure and will come up with jagged tempo info because it is not allowing tempo to change from within a measure, which could be good in some situations. If you wish to have a smooth ritardando or accelerando, you may want to turn this off and allow tempo to be changed within the measure.