Modify the transition behavior between two frames

According to AnimationClip node, the transition mode between two key frames is a property of the left key frame.

Therefore, to be able to modify one or more interpolation modes you need to select the proper key frame. The corresponding interpolation lines display in bold to be easily identified.

The information area over the timebar shows the current interpolation mode for selected key frames.

After the selection is made, you can change the interpolation mode by pressing one of the interpolation mode buttons over the timeline. The new interpolation mode instantly updates to reflect the scene.