How to change ZIP/Postcode to Post code label from the checkout page in Magento

At the billing address and shipping address steps of the checkout process in Magento the label for zip/postcode to change Post code. If you want to change, you have to make a few code modifications.

You need to go to public_html/app/design/frontend/base/default/template/checkout/onepage/billing.phtml. This path is to the billing.phtml file of the default theme that comes prepackaged with Magento; if you use another theme you may need to edit the file of that theme. In the billing.phtml file find and comment out the following code (around line 105):

<div class="field">
<label for="billing:postcode" class="required"><em>*</em>
<?php echo $this->__('Zip/Postal Code') ?></label>
<div class="input-box">
<input type="text" title="<?php echo $this->__('Zip/Postal Code') ?>" 
name="billing[postcode]" id="billing:postcode" 
value="<?php echo $this->escapeHtml($this->getAddress()->getPostcode()) ?>" 
class="input-text validate-zip-international <?php echo $this->
helper('customer/address')->getAttributeValidationClass('postcode') ?>" />
</div>
</div>

After that edit the file  

public_html/app/design/frontend/base/default/template/checkout/onepage/

shipping.phtml 

and comment out the code (around line 96): 

<div class="field">
<label for="shipping:postcode" class="required"><em>*</em>
<?php echo $this->__('Zip/Postal Code') ?></label>
<div class="input-box">
<input type="text" title="<?php echo $this->__('Zip/Postal Code') ?>" 
name="shipping[postcode]" id="shipping:postcode" 
value="<?php echo $this->escapeHtml($this->getAddress()->getPostcode()) ?>" 
class="input-text validate-zip-international 
<?php echo $this->helper('customer/address')->
getAttributeValidationClass('postcode') ?
>" onchange="shipping.setSameAsBilling(false);" />
</div>
</div>

You need to edit another billing.phtml file but in a different location. 

With public_html being the root Magento directory on your hosting account and 

with the default theme, the path to the file will be 

public_html/app/design/frontend/base/default/template/persistent/checkout/

onepage/billing.phtml. 

In that file find and comment out the code (around line 106): 

<div class="field">
<label for="billing:postcode" class="required"><em>*</em>
<?php echo $this->__('Zip/Postal Code') ?></label>
<div class="input-box">
<input type="text" title="<?php echo $this->__('Zip/Postal Code') ?>" 
name="billing[postcode]" id="billing:postcode" value="<?php echo $this->escapeHtml
($this->getAddress()->getPostcode()) ?>" class="input-text validate-zip-international 
<?php echo $this->helper('customer/address')->
getAttributeValidationClass('postcode') ?>" />
</div>
</div>