First, ensure that you've set up the theme correctly by following the install guide.
By following that guide you will have installed the Crelly Slider plugin and added the demo slider.
Step 7 of the guide is very important - you should have imported the demo slider file. The zip file is downloadable at the bottom of the install guide page.
Click on 'Crelly Slider' on the bottom left of the WordPress dashboard and if you've imported the demo slider - you'll see the Homepage slider to edit.
Click 'Edit Slides'
Step 3 - Changing the background image
Slide One will have been pre-selected. You'll see the other slides arranged as tabs on the top.
You can change the background image by clicking on the 'Select image' link and uploaded a new image to use.
You should always ensure that background size has cover selected - this ensures that the image will fill the screen and not be cut.
Step 4 - Changing the text
Scroll down and click on one of the text elements. It will be selected when you see a faint border around it.
Scroll down further and you'll see the text editing field. You can change the text here.
You can repeat this same process for the other text elements.
It's also the same for the other slides, as you've seen in Step 3 the other slides can be selected by clicking on their tab. Then, follow the same procedure.
Step 5 - Don't forget to save!
Ensure that you've clicked 'Save Settings' at the very bottom after editing each slide.