Magento 1.9 - PLP Page sort by ASC order

For Temporary:

Paste below code into Backend Magento category -> Custom Design -> Custom Layout Update section:

 
 <reference name="product_list">  
      <action method="setDefaultDirection"><dir>asc</dir></action>  
 </reference>  
  

For Permanent :

 Add following code to app/design/frontend/default/theme/layout/local.xml

 <?xml version="1.0"?>  
 <layout version="0.1.0">  
   <!-- Change default direction for simple searches -->  
   <catalogsearch_result_index>  
     <reference name="search_result_list">  
       <action method="setDefaultDirection"><dir>asc</dir></action>  
     </reference>  
   </catalogsearch_result_index>  
   <!-- Change default direction for advanced searches -->  
   <catalogsearch_advanced_result>  
     <reference name="search_result_list">  
       <action method="setDefaultDirection"><dir>asc</dir></action>  
     </reference>  
   </catalogsearch_advanced_result>  
   <!-- Change default direction for a category without layered navigation -->  
   <catalog_category_default>  
     <reference name="product_list">  
       <action method="setDefaultDirection"><dir>asc</dir></action>  
     </reference>  
   </catalog_category_default>  
   <!-- Change default direction for a category with layered navigation -->  
   <catalog_category_layered>  
     <reference name="product_list">  
       <action method="setDefaultDirection"><dir>asc</dir></action>  
     </reference>  
   </catalog_category_layered>  
  </layout>