By default, each object you draw or add to the canvas is placed on “top” of the previous ones. As objects stack up on each other, you may need to move things to top or bottom.
In order to edit objects “below” the top object, you may need to change the display order with the two commands; Bring to Front and Send to Back.
Select an Object
Tap the More icon in the lower left corner of canvas
Tap either Bring to Front or Send to Back to change its display order