I am now running Ubuntu 15.04 latest version on my Dell laptop(Inspiron 15R).
I find this version almost unstable, and not really good for my work. I want to downgrade this version to 14.04.
How should I proceed?
This can be done, albeit with a lot of work - so is not really worth it.
Your best bet is to install 14.04 from scratch - ie. a fresh install.
However, if you really want to do a downgrade, take a look at this answer.