For slider image change, You have to follow below steps.
Step 1. Go to Dashboard >> akhada-fitness-gym-pro Slider
Step 2. Click on Add New button.
Step 3. Add the slider name.
Step 4. Add slider button link.
Step 5. Add the featured image as a slider.
Step 6. Click on ‘publish’ button at top right of page. And Enjoy the Slider.
Here is step by step complete theme documentation. You may follow it.