if you want to change the x axis label , which probably is coming as 'DefectCode' you just can edit the chart type and put a custom x axis label to TA. Visualization > Format > X Axis > Ttile > Custom > (your label.)
However, I think the DefectCode field value is TA which you want to rename as Styles
For this you need to :
|eval DefectCode=if(DefectCode="TA","Styles",DefectCode) BEFORE the stats command.
Now, if you have other DefectCode types you need to append the labels you want after the last , in the if statement