<script>
function setValue() {
document.querySelectorAll('div.variant-option select').forEach(function(dropdown){
dropdown.selectedIndex = '1';
Y.one(dropdown).simulate('change');
});
}
window.onload = setValue ;
</script>
The above code can put in `Page Header Code Injection` and it will select the first variant option and displays its price as well
I modified @DomBrady answer to work with any product variant without having to specify variant details in code