To Format an Axis Expression
1. On a plot, select or click an axis expression.
Selecting a plot region applies the formatting to all axis expressions within that plot region at once.
Selecting one or more plot regions applies the formatting to all axis expressions within all the selected plot regions at once.
2. To format the selected expressions, on the Math Formatting tab, in the Math Font group, change the Font, Font Size, Font Color, or Highlight Color. The selected expressions change accordingly.
Alternatively, you can click the Increase Font Size or Decrease Font Size to increase or decrease the font size of the selected expressions.
3. To remove the applied formatting, click 
Remove Format.
Remove Format.