You cannot, and that is mostly a good thing. However, you can change
tracking using the Inspector > T > Text > Spacing > Character. And that is probably what you want to do anyhow.
The following paragraphs are just there to get the terminology right. If you do not care about the terminology, you can skip the rest of this posting.
Tracking is a fairly simple thing. It is just the space between the characters.
Kerning, on the other hand, is a property that is built into the font itself. It defines how certain characters are adjusted when they stand next to each other.

The two bottom fields in the picture above show kerning and tracking in Adobe InDesign. Here the kerning is shown as -37 within brackets - the default value for kerning between W and A in this font. Nothing is changed by the writer. The tracking is 100 - also the default.
As W and A lean in the same direction, they can stand slightly closer than for exampel W and T without touching each other. -37 expresses how much closer they can be according to the font designer.

Between A and T, the default kerning is -74. The font designer decided that they could get even closer to each other.

However, between T and E, the default kerning is 0. There would be no advantage pushing them closer together, as they already are very close on the top part.
Usually the default values are good, and you would not like to change them.
However, if you really feel you need to change the kerning instead of the tracking, you can use TextEdit. To see how it works yourself, create a new document with just two lines:
WATERHighlight the text and go to the menu Format > Font > Kern > Use Default. Now look carefully at the letters and go to Format > Font > Kern > Use None. You will see that WA changes, but not TER, where there is no default kerning.
The same trick does not work in Pages, where the similar menu is Format > Font > Tracking, and the options are None, Tighten and Loosen. This is just a round about way to do the same thing as in the Inspector - tracking.
Pages adjusts tracking - not kerning. And to most of us tracking is just what we need.