I have Drupal v7 and installed the Simplenews module, but now when I send a newsletter, I need to edit the HTML format.
Where can I edit this template?
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from this tutorial website I found after a quick google search: http://www.wdtutorials.com/2012/07/06/drupal-7-how-create-newsletters-simplenews#.VCEqY_l_uVM
There are 4 template files you can override in sites/all/modules/simplenews/theme:
Just copy them to your theme templates folder (like themes/bartik/templates) and edit them.
Once you've downloaded and installed the Simplenews module, your best bet is to also download and install the Mimemail module, which will allow email configuration to be set to use HTML rather than plain text.
Once Mimemail is installed, in its directory you should find a *.tpl.php file - this is the template you'll need to copy and drop into your theme among the other template files.
Edit that copy in order to change the HTML output of your newsletter email.