How to upgrade Joomla 2.5 to 3.1?

Step 1:
Make sure you have a current backup of your site.

Step2: 
PHP 5.3.1 is required for joomla 3.x
Check these in php.ini file.
1.    register_globals  must be off
2.    magic_quotes_gpc  must be off
Supported Databases:
1.    MySQL (InnoDB support required) 5.1 +
2.    MSSQL 10.50.1600.1 +
3.    PostgreSQL 8.3.18 +
Supported Web Servers:
1.    Apache(with mod_mysql, mod_xml, and mod_zlib)  2.x+

Step3:
The recommended way to upgrade Joomla version is to use the Joomla Update component found in the Components menu of your site Administrator. This will show an update indication. If it is showing joomla is up to date then go to the site maintenance option and clean all cache an purge cache and global checkin. Now refresh the page.
it will show the update indication.

Step4:
Click at this icon you will get this screen .

Here if Joomla update will show your current joomla version2.5.x and Latest joomla version 2.5.x. But you want to upgrade your joomla to 3.1 vesion . Then go to the options .

Step5:  

Select Short term support from Update server option and save it. It now show  latest joomla version is 3.1.5. Even if it’s not showing 3.1.x then clear all website cache and also go to control panel and click on Updates are available option and purge cache. After that open Joomla update component and it will surely show latest 3.1.x version.

Now click on Install the update option. And wait until the updation process completion. It may take several minutes.

If it shows  Error 500 - JHtmlBehavior::mootools not found. This means current framework does not support this update. Need to change the  framework

After successful update Joomla version is now 3.1.x.

Note- Many of the errors can occurs Because of your installed component, plugins and modules which are not compatible with joomla 3.x.  You Can replace those or you an remove errors.

IMPORTANT-
1-> When updating has completed you may need to refresh your screen or empty your browser cache to adjust for template CSS changes.
2-> Joomla! will notify on your administrator home page (control panel) when an update is needed, but it will not do the update for you. You need to press the button to start the update.

How to upgrade joomla 1.5 to 2.5?

Following important initial steps are required to upgrade joomla.

• Create backup of your website and database.
• Check your site 1.5 update version of joomla 1.5.26 or above.
• Check which 3rd party component used in your site and what will need for the new site. If you have extensions on your 1.5 site that you don’t need or care about, it is best to not reinstall them on the new site.
• Check your template purchase or custom build. If your template is purchased from other template site please check template are available in 2.5 or 3.0 on that site. Otherwise your developer will need to re-create your custom template.
• Create a new database on the new site server.

Now you can use jupgrade component to upgrade joomla.
You will need to download the jupgrade component from http://redcomponent.com.
Now this component have to install on your website following steps are required.
You will go in administrator->extension manager->install/uninstall.




Now you need to enable the Mootools upgrade plugin.
Path- Extension manager->plugin-manager->System-Mootools-upgrade.



Now you will select component->jupgrade and click.
Jupgrade Basic configuration page will open.



Now you will select parameter button on right top of page.



Set your required parameters which describe in popup window.
Now you will select first page of jupgrade page and click on Jupgrade button.




Note : You will must be install mod_curl of php for upgradation of joomla 1.5 to 2.5 .

After completion of upgradation you will got a upgrade finished message. Now your 2.5 site has been populated your 1.5 website’s articles, categories, menus, menu items, core modules and plugins, languages and users.