Traffic is removing its settings if you try to modify road with Road builder, otherwise it would leave junk data which could break further modification with the mod tool, cause weird resets or game crash.
By forcing certain priority on the lane you select with the tool
Can you elaborate what is not working? Can you record some short clip?
I thought that priority tool is so simple it doesn't need a guide or manual to use. Select intersection, then select type of priority (priority, yield, stop, reset) and simply click on the lane you want to apply it. Done. If you need more info, every button on the UI includes tooltip with short description what it does.