Magento – Rename add to cart to buy now

Kindly open app/design/frontend/package/theme/template/catalog/product/view.phtml

Add below line into <form> Tag

<input type="hidden" name="buy_now" id="buy_now" value="" />

Put below button at anywhere where you want

<button type="button" onclick="jQuery('#buy_now').val('buy_now');productAddToCartForm.submit(this)" class="btn btn-block btn-express-buynow"><i class="cart-icon-white m-r-sm v-middle"></i>Buy Now</button>

Now open app/code/core/Mage/Checkout/controllers/CartController.php

First of all move this file into the local folder and change the code as below instructions

Put below line in _goBack() function at very first

$buy_now = $this->getRequest()->getParam('buy_now');

FInd the line $this->getResponse()->setRedirect($backUrl); and replace this line as below code

if (!empty($buy_now)) {
    $this->_redirect('onepagecheckout'); // If you are using onepagecheckout or use this $this->_redirect('checkout/onepage/')
}else{
    $this->getResponse()->setRedirect($backUrl);
}

Code Taken from this link http://chandreshrana.blogspot.in/2016/01/how-to-add-buynow-button-on-product.html

Please add this in your phtml file where you want to add :

<div class="add-to-cart">
        <div class="qty-view">
        <?php if(!$_product->isGrouped()): ?>
        <label for="qty"><?php echo $this->__('Qty:') ?></label>
        <input type="text" name="qty" id="qty" maxlength="12" value="<?php echo $this->getProductDefaultQty() * 1 ?>" title="<?php echo $this->__('Qty') ?>" class="input-text qty validate-greater-than-zero-qty validate-digits" />
        <?php endif; ?>
        </div>
        <button type="button" class="button btn-cart" onclick="productAddToCartForm.submit(this)"><span><span><?php echo $buttonTitle ?></span></span></button>
        <button type="button" title="Buy Now" class="button btn-cart" onclick="setcheckoutLocation('<?php echo $this->getAddToCartUrl($_product) ?>')"><span><span>Buy Now</span></span></button>
        <?php echo $this->getChildHtml('', true, true) ?>
</div>

and must be added this script for Buy Now button :

<script type="text/javascript">
function setcheckoutLocation(location)
{
    showLoadingAnimation();
    jQuery.ajax({
        type:"GET",
        url:location,
        success:function(data){
            window.location.href = "<?php echo Mage::helper('checkout/url')->getCheckoutUrl(); ?>";
        }
    });
}
</script>