The following guide will show you how to add and edit the slides and the text on them:

They will follow the same process:

1. Go to admin -> promotion -> slide
2. Add New
2a. Setup Tab(Title, Order Weighting, Brand should be null, Category should be null, live should be checked)
3 Add Image
4. Once added go to the main website and hard refresh(ctrl + F5) 
You will get your slide displayed.

NOTE: No more than 10 slides can be displayed

Also, please find below the answers for:

How can I change the order they display in?

There is a weight field (order weighting) in the slide area you can change that to change the order (1 shows first and 10 is last)
How do I change the text (currently it is set the same on every slide and seems to overlay the image as):
There is a text/button tab if click on that it will you options to change the text showing on the banner as well as the button and the option to add a link to where the button will go.