* This Tutorial should help some Joomla Users with Upgrading their sites from Joomla 2.5 to Joomla 3.4.8
Note - Always Backup your site before making any updates.
Should you require a backup tool I would recommend using Akeeba Backup. Its an excellent tool used by thousands of Joomla users - extensions.joomla.org/extension/akeeba-backup

Tutorial
To Update your site from Joomla 2.5.8 to Joomla 3.4.8
1: Download the required zip file from - github.com/joomla/joomla-cms/releases/tag/3.4.8
2: Login to your Joomla 2.5.8 administration
3: Goto the Extensions Manager and upload the update zip file you downloaded from Joomla. This can take a few minutes to upload.
4: When the upload is completed you will be logged out of the administration.
5: You should see the Joomla 3 administration page now. Log back in to your administration to see all looks right.
6: Check the site from the frontend. If you see a blankpage after updating it is probably due to one of the following issues.
- The Joomla Template you were using before is not Joomla 3 compatible.
- The Modules/Extensions you have installed are not Joomla 3 compatible.

To Update your Joomla Template and Modules after updating your site from Joomla 2.5.8 to Joomla 3.4.8
Note: I do this in Cpanel file Manager or through Ftp. Use whatever suits you best.
1: Navigate to the folder you have your Joomla installation in.
- If your site is at Example: www.mywebsite.com then your Joomla Installation would be in your Public_html folder.
- If your site is at Example: www.mywebsite.com/joomlasite then your Joomla Installation would be in your Public_html/joomlasite folder.

Updating Template to Joomla 3 Version
1: In your Joomla website installation navigate to the /Templates folder.
2: Delete the existing template.
3: Now upload the Joomla 3 version of your template to the /Templates folder.

Updating Modules to Joomla 3 Version
1: In your Joomla website installation navigate to the /Modules folder.
2: Delete any existing modules you had installed in Joomla 2.5.
3: Now upload the Joomla 3 versions of the modules to the /Modules folder.

Open your Web Browser and now check your Joomla site. Is it loading properly. Fingers crossed it is.
Note: You may need to clear your browsers cache a couple of times to refresh the site properly. Do this by holding down CTRL-R or by pressing the F5 function key.

Good Luck with your updating.