The slide items in WordPress posts are ordered by date and time they were published, similar to how all WordPress posts are ordered.
Change Slider Post Order Manually By Post Date
To change the order of these items you will have to edit the slider item and change the post date to earlier or later depending on where you want a specific slide to display in the order.
Edit the publish dates on this screen below.
Change SliderPost Order Using The Post Types Order WordPress Plugin
If you want an even easier way to achieve this you can use this plugin below.
Screenshot of where to edit slide post order via drag and drop after installing this plugin.