You must have created a new Page with name : About us before.
Go to Pages -> Add Page -> Fill in Title, content and Choose Template -> Save
!!! Please make sure that the Handle is about-us
Go to Customize to edit the page
You can also using Section to edit About Us page
There are 3 blocks in About us: Text, testimonial and member
You can change location of them.
** Add Text
Go to Section -> Text columns-> Add columns
** Add Testimonials
Go to Section -> Testimonial-> Add Testimonial
** Add Members
Go to Section -> Members-> Add Members