You can change the theme by using the default themes or by downloading it.
To apply the default themes:
Type http://localhost/drupal in URL of any browser
Go to Adminster> Themes.
Choose any theme from the list by clicking the check box Enabled and Default.
Then click on button Save Configurations
The theme will be applied.
Now if you want to apply any other theme. Then download any free theme from the site www. drupal.org
Choose any theme and click on download e.g. Deco.tar downlaod
When it is completely downloaded. right click on it and choose Open Containing a Folder.
Choose those folder and copy it.
Now create any folder on the desktop e.g. Drupal site. In it again create a folder e.g. Theme . Now Paste that copied folder in it. Right click on that folder and choose Extract here. now in addition to zip folder, You will be having a simple folder of that name.
Now go to Terminal and type gksudo nautilus
Root dialog box will be displayed . Go to Filesystem>var>www>Drupal>Themes.
Now go to Drupal site folder on the desktop. Choose theme folder then choose that downloaded simple folder, copy it and paste in FileSystem>var>www>Drupal>Themes
Again go to http://localhost/drupal, Administer>themes.
The newly downloaded theme will be present in it. choose that theme and click on save configurations.
You are done with applying the Theme……:-)
For further Help, Have a look on the video from whom I was guided.