Depends on what page you want to edit.   But html pages are typical located in the follow ways: 

ow_themes/yourtheme/masterpages    you will have a dndindex and a general html page 

also for other pages refer to the plugins html pages which are located as such

ow_plugins/the plugin name/views/controllers/

there are also some for the base of the software

ow_system_plugins/base/views/controllers

there are other files in other places but they are typically used for widgets and components. 

hope that helps 

also remember never never never use a word processing editor to edit html or php or js or any kind of computer code (it leaves hidden special chars in the code and trust me you dont want that mess, you will be pulling your hair out wondering whats wrong with your coding).  Use notepad or notepad++ or programers notepad  

And anytime you copy from a webpage, always!!!!!!  past it to notepad or one of the others i mentioned before you paste it in your code file. 

Hope that helps  

Thomas , +1 to Dave's replies.

Which particular page would you like to edit? Which element on the page?

Thanks for the advice.