How do I add custom images to slider and delete default images permanently

How do I add custom images to this slider and delete the default images? Everytime I add an image and delete the default images they just reappear again once I visit the site.

I am using the Planshet theme. The problem is there is no "save changes" button after I do the image work. So how do I know the changes have taken effect? Clearly the image changes I am making are not taking effect.

To save changes you have to click a check mark on the right.