First time redesigning an existing Squarespace website

Hey,

I recently booked a project to redesign an extensive Squarespace website. My client has a shop and about 10 pages. I have only built brand new websites and many of them. I don't want to mess up her content or have this project take forever. 

I am pretty sure that her template is on Squarespace 5. I believe she said it was "Devlin" - which no longer exists.

What series of steps from start to finish would you take to complete this project?

And are there any time saving tips?