What OS and version of PS?
Change a number on image? This a text you have to change such as 4555 to 5444?
its CS4 and yes, I need to change the number 345 to 425
Is the document made of multiple layers?
If not then:
1. Save As PNG
2. Attach the PNG file to a reply to this message so someone can change the number for you.
If hes never used PS I doubt he knows what layers are or how to save as a png.
This reminds me of a time when our graphics department decided to take a day off then I was recruited to learn Illustrator in one afternoon to fix a graphic. Miraculously I did it, but I don't know how since I really didn't know what I was doing and never used Illustrator before then.
I'm sure there are some tutorials at this site that would help, but I'm assuming it's too late now. Here is a Photoshop CS4 manual in PDF format available here:
http://help.adobe.com/archive/en_US/photoshop/cs4/photoshop_cs4_help.pdf It might be a little too much for the project you're working on now, but it will show what the layers are and other information about the version of PS you're working with.
The quick and dirty method would be to just erase the old number and replace it with a new one provided you know the font. With a PS file with layers, the obvious would work. Just select the text and replace it. The layers box is on the right side of the screen unless it was moved or not showing.
I agree with Conroy. Send the image to this forum so somebody can fix it for you instead of laboring over it all night and still not getting it fixed. Then go home and read up on PS for the next time they stick you with another project because they're going to think you're an expert now. That's what happened to me in my little story above.