HTML edidting

Hi to all.

i am very neew to modx and having a problem with finding my way around the system and even aroud this forum so if i posted this post in wrong place please forgive me.
My problem is as follows:
i would like to make some changes to one page within modx, troubles i am having is that i dont know how to change HTML code to just one of the pages so it will not effect the other pages. i know how to modify the whole template but within the template there are 14 pages and i dont want to make changes to all of them only to one of them, any help will be appreciated.

Many thanks,

Make another template and assign the page to that template.