Here is the code :-

<template id="survey_fill_form_start_inherit" inherit_id="survey.survey_fill_form_start"
name="Survey Fill Form Start Inherit">
<xpath expr="//div[hasclass('o_survey_start')]//div//button[@value='start']" position="replace">
<button type="submit" value="start" class="btn btn-primary btn-md" >
<t t-if="survey.certification">
Start Certification
</t>
<t t-else="">
Start your_survey_name
</t>
</button>
</xpath>
</template>