I don't think you can get there with Astro. I use Root Explorer to go to /System. The file exists in the System folder. I think you can use ESFile explorer also to get there. To edit or rename a file there, you must mount /system as RW (read/write). If any changes have been made to the build.prop file, you will receive the error message you got. When I edited the file in Root Explorer, it kept a copy of the original as build.prop.bak in the same folder. I just deleted the edited build.prop and renamed build.prop.bak to build.prop. Then I flashed with no problem.
I found the build.prop file but no backup file. Guess Astro did not search the system directory or it would have found it the first time. Anyway build.prop says it was modified on the 9th of January 2012. That may have been when I rooted the phone. I did a back up when I did it. I don't suppose you can do like on the PC restore a single file? Sure am glad you gurus are around. Thanks
Chuck