To replace the text under Fig.1, highlight This is a caption and replace text with your caption. You can easily change the figure size by clicking on the image and changing the size according to pixel width (or Small, Medium, Large). Additionally, if you want to replace this figure with another one, simply hit replace and upload a new image.
Pretty standard stuff. But wait... Here's where it gets cool.

Data and code "behind" figures

Because Authorea documents are not just flat layers of text but actual repositories that means you can do some really cool stuff with images, like adding the data or code that was used to produce the image. Click on an image and select Attach data to choose whatever datafile you'd like to attach (.csv.xlsx.txt). A data icon will appear directly next to your image, thereby helping you and your collaborators easily manage and share data. See Figure \ref{919085}, below? It has a data icon to the left of it. Click it to show the data behind the figure. 
(Oh, did you notice how we dynamically referenced the Figure? Yes, it says "Figure \ref{919085}", not "Figure 1". You can reference any part of the document by Inserting a Reference from the toolbar ❡ - it is the best way to keep track and manage references to figures, tables, equations, sections)