Changing sprites to yours

Images are pretty easy to edit

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UI

Select any UI element from the [UI] object on the scene. Then in the Inspector window find the Image component, and in the Source Image box drag/select the desired sprite.

Important!

In order to be able to use an image in the UI, it must be Sprite. To convert the image into a sprite, you need to select it in the project and in the inspector window in the Texture type item select Sprite

Level

The game level is made with the Tile grid system. To work with the level, you will need to open the tile editing window: Window->2D->Tile palette

More details: https://docs.unity3d.com/Manual/class-Tilemap.html