WordPress is very easy to customize, especially to modify the design and change or install new themes.
How to install theme in WordPress
There are two methods for installing new themes in WordPress: automatic and manual.
Automatic theme installation in WordPress
1) Click on the Appearance link on the left side of the Dashboard
2) Select Install Theme tab on the top of the page
3) Search for themes by keywords or find a theme based on specific features
4) Click Find theme button
5) Themes that meet your search criteria will be displayed
6) Click Preview link below theme's thumbnail in order to preview it, the Details link to see more details about the theme and the Install Now link to install it
7) After the installation, activate the theme and your website will have new design.
Manual theme installation in WordPress
1) Copy your theme's files into /wp-content/themes folder
2) That's it, the next step is to activate and change theme in the Appearance section. Read below how to do this.
How to change theme in WordPress
1) Click on the Appearance link on on the left side of the Dashboard to choose a theme.
2) You can click on the theme to see a preview of how that design will look with your content.
3) Once you have selected a theme click on the Activate link on the top of the preview window or click the Activate link at the bottom of the description.
4) Depending on the theme, you can also modify the header image and load a new one to give your blog a unique look and feel.
You can do this through the Appearance menu dropdown. If you have configured custom menus or widgets, you will have to re-add them after changing your theme.
Different themes will often have theme specific options and theme specific widgets so you could configure them selecting appropriate section under Appearance menu.