Change Resolution in Photoshop

I photoshopped a pic and resolution was set only on 300 pix/in. now my client wants to enlarge it and the pic is blurred. Any ideas how to do damage control and change to higher resolution like 1200? I did lots of edits on it don’t wanna hafta redo everything!

you cant really properly enlarge it afterwards.
you can try going to image-image size and play aorund with the resampling choices and see if any give you a better final result