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:
http://hostlantern.com/clients/knowledgebase.php
Then click on the Default icon besides the chosen template.
Joomla is a powerful software that is easy to work with once you have the right host to support you. Before you lose any more time with the wrong host, check out the hostlantern Joomla hosting offer.