Update Joomla from 2.5.28 to 3.4.0 - php

Question

I need to update my joomla website which current version is 2.5.28. Now I'm trying to update it using this:
I use XAMPP as a server environment (PHP 5.4, MySQL 5.6)
Use a backup of my site
Also use astable version of joomla 2.5.28
I tried to update with both sites, the stable one and my one, but every time I get errors.
If I want to update my site I do this:
check for updates and update all things but for the JSN air template
go to joomla update, click on options on the right side and choose short time support, save and close
click update button
With the stable site I nearly do the same:
go to joomla update, click on options on the right side and choose short time support, save and close
click update button
I said above that I get errors. If I want to update the stable one I get these results/errors:
after updating it completely (100%) I get a white page with an url like "http://localhost/stable/administrator/index.php?option=com_joomlaupdate&task=update.finalise"
if I reload the page after getting the white page I get an error like "Can't DROP 'usertype'; check that column/key exists SQL=ALTER TABLE bcyqr_users DROP KEY usertype;"
if I reload the page again I get an error like "Table 'stable.bcyqr_postinstall_messages' doesn't exist SQL=SHOW FULL COLUMNS FROM bcyqr_postinstall_messages"
I looked in the database and I have a column with name "usertype". What happened? Why it can't be deleted?
I think it's strange that it doesn't function with the normal joomla settings. On youtube I saw videos in which the people just did the steps I also did.
And with my site I get the following errors:
18 times: Use of undefined constant DS - assumed 'DS' in C:\xampp\htdocs\mysite\administrator\components\com_poweradmin\defines.poweradmin.php
1 time: Class 'JSNConfigHelper' not found in C:\xampp\htdocs\mysite\plugins\system\jsnpoweradmin\jsnpoweradmin.php
I googled that error, so I used this extension:
http://digitaldisseny.com/en/joomla-extensions/solve-undefined-ds-constant-joomla-3-error
But then I couldn't update the joomla. I get a popup dialog with message "Invalid login".
I don't know what to do to update the joomla. Can you help me? What I do wrong?
Thanks.

Answer 1

Joomla 2.5 to Joomla 3 is a mini-migration, despite the fact that Joomla officially labels it as an update. We have migrated many sites from 2.5 to 3 and never were we able to update the site from within Joomla.
What we do is that we create a fresh copy of the latest version of Joomla, and then migrate the data there from 2.5.

Answer 2

I found this alternative guide which worked for me. I had tried the official and various other guides but this seemed useful
It claims
The following method avoids the pitfalls that occur due to third party extensions
It apparently works by bypassing the files of extensions and prevents them from interfering with the population of the database changes.