In a Canvas Block you have the ability to place Blocks on top of each other. Below there are three Text Blocks in Canvas Layout that do just that.
You have the ability to either to take any Block in the Canvas and "Bring to Front" or "Send to Back". This could be to either change the Layout or simply access the Block to edit it and move it back to where it was
To move a block to the Back or Front simply right mouse click on the Block that you would like to move and choose "Send to Back" or "Bring to Front".
In this example we brought the Text Block "Item 2" to the front.