Poor cassio watch display

Did you "reset" the movement when you changed the battery?

In most Casio's, there is a small contact recessed in the top "pallet", that is, the 1st level of the movement. It will be labelled, "reset" if it has one. You will have to get a thin metal object, - a bent pin will do- & touch the contact, very lightly, & just for a second with the + side of the battery.
Good luck, let me know how you go.

You don't say how old the watch is and whether it has always had a display problem. I have seen several watches with weak displays, but generally they are older watches and I suspect the problem involves a corroded contact somewhere in the "movement". You might want to visit the Casio website and download a copy of the manual for your watch. It may say something about the display. If you watchis fairly new it may still be under warranty and again, contact Casio to see what you need to do.