In screen blocks you got
set titlebar back button blocks set it to true
You can apply the technique told by @Faraz_Firoz or use this -
Add an label component and select material icons from font type and type
arrow_back in the text. Adjust it according to you. Now your are done
This might be helpful
If you are creating custom title bar then set button font typeface to material icon and button text to keyboard_backspace.