To split the street, select the street and while holding down the shift key, select the node where you want to split  the way. The right click and select the scissors to split the way. Then you can update the relation tags for the bus route you want to change. Let me know if you need help. I usually do most of my editing using the JOSM editor which makes editing transit routes easier. But JOSM has a somewhat higher learning curve. (At least for me). You can also reach out for help on Slack, osmus.slack.com to join.