Up gradeing tp PHP5 for plugin

Hi,
I have to upgrade to PHP5 for a plugin I need to use. I got onto my host and this is what they said,

You can do so, by creating a .htaccess file in the plugin’s folder, and in it insert the following command:

SetEnv PHP_VERSION 5

How do I do this. I have seen 3 plugin files, plugin editor, plugin install and plugin phps
Or do I just make a new .htacess file, put in the command into the same folder where the plug ins are.
Problem is they have also said,

I have consulted with our administrators, and they have confirmed, that the modification will work only for specific folder (or subfolder), which means that you will be able to use it just for your plugin. In addition, you will have to do the same modification for each domain/subdomain/plugin, you wish to use.

So by reading that it really throws me, as it looks like upgrading will only work for one plugin. I can not see where the individual plugins files are.
Can any one help me and send me in the right direction to do it,

Thanks