render onto existing texture

I’ve unwrapped my obj to a texture. Now I want to bake ambient occlusion into THIS TEXTURE. How? It always creates a new black image but doesn’t use the existing.
Many thanks

Let me propose a deal: make an effort to use the correct terminology to ask your question and I will answer it in detail.

what you mean?

there should be a “clear” check box in the bake panel. uncheck it.