How do you change the X axis in Numbers?

Can you be more specific about what you're trying to do? As written your question can be interpreted multiple ways.

For example, the 'obvious' answer is to change the values in the cells that feed the chart, but that seems too obvious.

SO maybe you're referring to the scale of the axis, or the number of steps/divisions show? That's controlled via The Axis sidebar when you select the chart:

Any other interpretation of the question will need more details.