How to edit .png file?

[Bob Goodwin <bobgoodwin wildblue net>]

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In updating this system from Fedora Core 5 to FC6 I lost a .dia file 
[sys.dia]. 

I did however have a .png copy on a server that I was able to copy.  But 
now
I find that there is no way to edit the text on my [sys.png] diagram!  
Is there some way
to use this file without manually re-creating the diagram?  I tried 
'convert' but
that doesn't seem to know about .dia?

Any help appreciated. 

Thank you.

Bob Goodwin
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