How to modify an intersection

Hello everyone,
I just started today with osm today, so I apologize if this is trivial. I want to export an osm file and modify if by changing a stoplight intersection to a roundabout intersection. I will import the modified osm file for a simulation I am doing. Thank you for the help.

Hi and welcome to OSM!
You can use JOSM (1) for this. If your simulation uses an OSM download as input, the workflow depends on the size of that file. Presuming you provide enough memory JOSM can load a mid-size city (50.000 people) but will be very slow when you load the data for a large well mapped city with > 250.000 residents.
If your area is too complex, download only the area that you want to change and modify it. Save the result to a file and merge the large input file with that smaller file. I think osmconvert should work as a merger.

What simulation are you doing it? Won’t it be more organic to modify the layout in the software you are using?

Thank you for the responses. I was originally using sumo netEdit, but sumo treats a roundabout as 8 intersections and then I would need to define to define the right of way. It is easier for me to just import the osm file with a roundabout to sumo. I have done that with another osm map I imported. I tried yesterday using the osm online editor, and I like that I could select a stop light junction and replace it with a roundabout in one step. I just got the josm to work, and will try to do the what was posted.