Changing your wp theme is not as hard as you would think. To do this just simple upload your new theme to the wp-content/themes folder - directory on your site then watch this short video on changing the theme.
To change the theme follow these simple steps.
Log in to the admin section of your wp-blog.
click on the design tab located towards the top left of the control pannel.
Your current theme is shown at the top and all available themes are shown below this.
Click on the theme you wish to use.
Then click on activate which is located in the top right of the graybox.
For a video click the link below this will open in a greybox.