It was kind of rough for me to really have my website upgraded to 2.5 joomla version from 1.6. However after reading a couple of links here and there these are my versioned steps to have your website upgraded to 2.5
Ensure your website is with version 1.6.5 (the last update in 1.6) if not please visit get the update from this link (link to stable 1.6.5 patch Joomla_1.6.0_to_1.6.5-Stable-Patch_Package.zip).
After downloading it, unzip and copy only directories/folders (Administrator, Libraries, language) as shown in the image below:-
Paste them into your website folder/directory. Confirm from your administrator panel version of 1.6.5 as per the image below.
If YES ......... great your good to go on step 2.
Upgrading from 1.6.5 to 1.7.0 joomla.
Very simple just download Joomla_1.6.5_to_1.7.0_Package.zip get a file from here. Unzip you will see two other zipped files as shown below.
Go ahead to unzip Joomla_1.7.0-Stable-Update_Package.zip file. Copy all those files into your website directory/folder. When you refresh you should see something like image below:-
Now you will need to upgrade your website to the last version of 1.7 which was 1.7.5 and download that file here Joomla_1.7.0_to_1.7.5-Stable-Patch_Package.zip. Do the same as previously, unzip, copy the files from zip then paste them into your folder/directory of the website.
Upgrading from 1.7.5 to 2.5.0
Go download Joomla_2.5.1-Stable-Update_Package from respiratory. Click here to download. After Unzip and copy all the content into your website folder/directory.
To confirm that website has changed version: Go to Site -> System Administrator -> System Information
Then go ahead to get the latest version of 2.5.x as shown on joomla website.
Now I know you will face a couple of problems with templates but each template gets a unique error message.
For example I have Yootheme template and I had to adjust one line of code in their WARP Framework
I used the answer on the link below to resolve my problem.