Change content width

Hi,

Thanks for a very clean and good-looking theme.
I am using a Child theme and am trying to change the content width as I’d prefer to have the text in a smaller frame.
I have tried creating and uploading a new page template (in the Child theme) but that template is not showing. I have reactivated the theme and the Child theme as well.
Alternatively, can you please suggest another way I can decrease the content width?

Thanks!
??