Here is how to change open cart theme.
1) Download the open cart theme form this site..
2) Open the download file and go on this address.
3) Copy the theme form this address.
4) Now paste this folder in your opencart folder. on this address.
6) Open opencart admin pannel.
7) Click on settings.
8) Select the store and click on edit.
9) Select store tab.
10) Select new theme form template.
11) Click on save.