Hi Tang,
You can' change the Icon of the search help as it is not available during design time in personas. You cant even hide just that. Its a actually not a text box but a different control and personans 2 really didn't provide any customization for that.
What karthik is suggesting above is to completely hide this control(Say O1), put a brand new text box(T1), Search Button (B2) and on click of B2 copy the contents of T1 and paste it to O1 and press enter, technically it should work.
The above image is little confusing as it was also showing the image you want to change but hope this explanation clarifies your question.