You need to select keyframes in order to edit them. You can select one keyframe at a time, marquee select a group of keyframes, or select all keyframes in a given channel.
Selected keyframes are yellow. Unselected keyframes are red.
To select a keyframe:

The keyframe is selected.
Once selected, you can right-click on a single key frame and modify its value with the on-screen calculator. It is also possible to enter a number on the calculator and use the up/down arrows to interactively modify the value.
To select multiple keyframes:

If you release the Ctrl key and then click a keyframe, this keyframe is selected and the previous selection is cleared.
To marquee select multiple keyframes:

Keyframes inside the marquee selection are selected.
To select all keyframes in the selected curve:
