You can replace all the image from images folder of template with same name and extension.
or
1. Change image from html
You can link your own image path to change any image in our template from src attribute of img tag.
<img src="your-img-path" alt="image-alt-text">
W3School can help you to find more in this page.
2. Change image from css
And for backgrounds, change backgound : url from your stylesheet.
background: url(your-image-path);
W3School can help you to find more in this page.