Linux Distributions are the UNIX like Office Software which includes a huge collection of software and database applications. Ubuntu is a free and open source operating system which is based on the Debian (uses Linux kernel) Linux Distributions. In Ubuntu Office Software you can change the default application to open a file if the wrong software program is appearing when you are opening it. You just need to right click on the file you wish to open and go to properties. Select the program from the list by clicking on "Open with" tab. If the required program is not available in the list then you can browse to select and add the program in the list.
When you double-click a file, a default action is executed.
(For example, if you double-click a .jpg file, it is the image viewer "Eye of Gnome" which is launched).
It is very easy to change the default actions.
Right-click on a file type (e.g, .jgp) and select "Properties"
Go to the tab "Open With" and choose the program to use (for example, The Gimp).
Note that you can use the "Add" button to use the program of your choice.