If you want to change the default template for your Joomla 2.5 web site you should log in the Joomla's admin area.
Navigate to Extensions -> Template Manager.
Select the one which you prefer or install a new one as described in the following knowledge base article:
Then click on the Default icon besides the chosen template.
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VN:F [1.9.22_1171]How to change the default Joomla 2.5 template?,