Hello 71, and welcome to Wikipedia! Thanks for the question. One of the most basic rules here is that information in articles must be "sourced" to a "reliable source," which is explained here: WP:RS. In the case of the Montana edit that you made, all the published sources list Joe's birthplace as New Eagle, Pennsylvania. If you have a published source that says Butte, Montana, you would have to insert it after the edit that you make. One way to do this is to click on the open book icon above the editing window to the left of "Advanced." A dialogue box will appear in which you can paste the URL for your source, or you can list the publication. The preferred method for sourcing, though, is listed here: WP:CT.

Unfortunately, insider information like knowing JM's son cannot be used as a source. Sources must be reliable and published, as above. I have some firsthand experience with this. One of my younger brothers is Terry Moran, and for several years the Wikipedia article had the wrong birth year for him. This was because the ABC News website had the wrong year - but I could not change the year and say "he's my brother and I know when he was born." I had to find a published source that confirmed the correct date. That's how things work here. I hope you'll enjoy editing article here at Wikipedia! Regards,