Your question is not clear. Did you register your block to start in the preview mode? Then, when you load the block, you need to switch to edit mode. Or, if you registered the block in auto mode, then simply click on the block to automatically change it to the edit mode. Are you talking about having default values? You do that in the Field Groups when creating the fields… I can go on and on…
P.S Clicking the pencil icon on the toolbar will change the mode to edit, and when you click on the Monitor icon next, you will go back from the edit mode to the preview mode.
Follow the example found on this page: https://www.advancedcustomfields.com/resources/blocks/
and continue reading this: https://www.advancedcustomfields.com/resources/acf_register_block_type/
I hope you can solve your issues !
