app > design > frontend > themename > subtheme > layout > page.xml

Edit

<block type=”page/html” name=”root” output=”toHtml” template=”page/3columns.phtml”>
Approximately Line 28 edit

and replace with

<block type=”page/html” name=”root” output=”toHtml” template=”page/2columns-left.phtml”>

or desired page template. The Default Templates are :

1column.phtml

2columns-left.phtml

2columns-right.phtml

3columns.phtml

Find the below given line

<button type="button" title="<?php echo $this->__('Add to Cart') ?>" onclick="setLocation('<?php echo $this->getAddToCartUrl($_product) ?>')"><span><span><?php echo $this->__('Add to Cart') ?></span></span></button>

And just replace it with,

<form action="<?php echo $this->getAddToCartUrl($_product) ?>" method="post" id="product_addtocart_form_<?php echo $_product->getId()?>"<?php if($_product->getOptions()): ?> enctype="multipart/form-data"<?php endif; ?>>
 <?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->getMinimalQty($_product)?$this->getMinimalQty($_product):1) ?>" />
 <?php endif; ?>
 <button type="button" onclick="this.form.submit()"><span><span><span><?php echo $this->__('Add to Cart') ?></span></span></span></button>
 </form>