i want to change "Date Book" to BOOKING NOW" at book a tour form

hello, i want to change the date book title to booking now at book a tour form area.

how to do that?
i have try this code
<input type=”text” name=”date_book” value=”” placeholder=”Date Book” readonly=”true” id=”dp1555084123742″ class=”hasDatepicker”>

and change to this code.
<input type=”text” name=”BOOKING NOW” value=”” placeholder=”BOOKING NOW” readonly=”true” id=”dp1555084123742″ class=”hasDatepicker”>

then, i paste at custom css . but then nothing happen.

please help