How to open *.MER (Rockwell Software RSView Studio runtime file), so I can open it with RSView STudion and make some modification on it and re-download again back to Allen-Bradley PanelView Plus
You didn't mention the version you have running, but assuming it is Version 4.0 or earlier then the easiest is to get the APA file. The MER file is not able to be changed to a MED file easily, and will quite likely lose parts of the file if you tried. If you are desperate, you could try running the MER file in ME Client, search for the MED file created and copy to somewhere, then look at editing this as well as look for the 'gaps' that happened with the MED creation.
Version 5 is supposed to support editing/importing MER files but I would expect this feature to work with Version 5 and later.