The easiest way is to open the route window. Pause the game. Click on the $ symbol next to the route you want to replace vehicles, this sends all of the vehicles to the depot and automatically sells them. It helps to name your depots so you know which one to pick to buy new vehicles. If you don’t there is a list of depots at the bottom of the routes window. When you click the $ the buses will appear next to one of those depots under “in transit”, or “on their way” (can’t remember the wording). Select that depot and then buy new vehicles, assign to the line (and you know which line if you keep the routes window open). Then un pause. This has the least effect on your route and doesn’t seem to affect ridership.
Definitely connect your towns together – people will travel and your towns will grow. The key is building the cargo lines to feed your towns not only passengers but also good. If you don’t connect them, the towns will not grow.