I am using TeXstudio 2.3 the last two years. Now new versions are available. I want to update the old version. Is it possible to update without uninstalling the previous version? Is there any way to get updates automatically like windows update? Any one can guide me regarding the two versions 2.3 and 2.6 which will be better?
You can simply install a new version on top of an old.
It's not recommended to install two different versions in parallel, because they will share settings. While we guarantee, that older settings can be read by newer versions, we do not test for the reverse. If you really need two separate versions, go for the USB ones. They store the settings in the folder next to the executable.
We have an update checker in TXS for a while now. It will notify you about new releases. However, there is no such thing like an automatic installation.
You can test the recent version without system installation: Download the portable version marked as USB type. Sigur gave you the link in comment already: http://texstudio.sourceforge.net/#download. Extract the ZIP file into a different folder than your existing installation and start the executable.
I do not know version 2.3, but I know, that version 2.6.6 (the recent subversion) is much better than 2.4, that I once tested. And also usually you do not have to uninstall the old version, when you perform an update, but you should see, whether some sort of a Readme file exists and tells a different story.