use window.location.href
to redirect
function changemap(select){
event.preventDefault();
var maps=document.getElementById('city');
if(maps.value == 'mumbai'){
window.location.href = 'bank_atm_locator_mumbai.html';
}
else if(maps.value == 'aura'){
window.location.href == 'bank_atm_locator_aurangabad.html'
}
else if(maps.value == 'pune'){
window.location.href == 'bank_atm_locator_pune.html'
}
}