Restore PI3 to factory settings w/o

I have successfully installed a image on my pi3. Now I can’t boot the PI with a normal Rasbian image anymore. I realized that I do not need and I just want to get back to normal again. The problem is that the pi doesn’t boot anymore with a normal rasbian image and it seems that I can’t flash any other images except for the images.
I have already delete the device but it did not help.

Can somebody point out to a step-by-step instruction how I can restore to factory settings?

I helped myself. Formatting the sd card helped. Afterwards I could fash a rasbian image and my pi was back to normal.