Installing a WordPress theme is very easy to do. However, it should be noted that if your blog is hosted by WordPress, you cannot use custom downloaded themes. If your blog URL looks like http://www.blogname.wordpress.com/, it’s hosted by WordPress, meaning you can’t install or customized the themes.
However, if you have your own domain and self-hosted blog running WordPress,
Here are ther process how to change your theme:
1.Connect to your server with an FTP client and navigate to your wordpress folder. You should see a bunch of files and folders, including wp-content and wp-admin.
2.Now navigate to wp-content/themes/ . If you don’t have any custom themes, you will see two folders there: default and classic. These are the two themes that ship with a default WordPress installation.
3. Download best theme for your blog & Unzip the file. It will contain a folder with the theme name, a readme txt file, and a license txt file
4.Upload the theme folder to the wp-content/themes/ folder on your web server. Make sure the upload finishes before you close your FTP client.
5.Log in to the admin area of your website (by appending /wp-admin/ to the URL like http://www.blogname.wordpress.com/wp-admin)
6.Now navigate to Appearance >> Themes.
7.The newly uploaded theme should be available under available themes. Click the theme link to use the theme.
8. There you can customized widgets, blog header, css editing etc…
So your blog is ready… So try now.
This guidline will get your newly acquired wordpress theme up and running on your blog in just a few minutes but mind it its only for wordpress blog not for blogger blog . If you want to know about blogger theme instation then please go throught it
Installing a Blogger theme on your blog.