How do I tell my boss I'm keeping my other job?

I'm working two jobs, the one I had before the other gives very low hours.. but I still like working there. The new one I got gives a lot more, so I told the boss of that one that I'd be leaving my other one. However, since I found out my new job isn't open on Sundays, I've limited my other job's schedule to just that day (my other boss is fine with it). This way, it won't interfere with my new job... but the thing is I told my new boss that I was planning on leaving, but I kind of changed my mind.

I told one of the assistant managers (cause we were just talking), and she didn't think it was a big deal that I was only working on Sundays. But I still need to tell my boss. So...

It's only been almost a week, would this still this look bad? How should I do this?

Thanks!