Hi there 
Before the start of a race there is a menu screen that displays sections such as the option to adjust the difficulty, to tune your pre-selected car, to leave the event and so on.
Currently selecting the car you want to drive happens before you get to the afformentioned menu in a separate one.
Once you have clicked on a car to select it, there is no going back to the car selection screen, from the menu that follows. Therefore, in case you have misclicked when choosing or have made up your mind to choose a different vehicle, you have to leave the entire event and start a new with the process of entering it.
Not only can this be a slight annoyance for some (as you might even need to find the correct event again, in case it is a community event) but this also means, that you have to wait for all the loading screens/loading times again, which might be especially unpleasent for players, whose systems have weaker hardware.
Furthermore this issue becomes all the more apparent, when you realise during an ongoing race, that you need to change out your car for a better chance of success, possibly even after multiple unsuccesful attempts with the one you have initially chosen or maybe when you quickly want to try and test what works best, without having to waste additional time.
As a long-time fan and player of the Horizon Series, I have to admit, that this is something that has bothered me for years now, even though it might at first appear to only be a minor issue in an overall great franchise.
Therefore my proposal for a solution to this problem, that might be a slight inconvenience to some, whilst maybe even leading to an unpleasent experience for others, is to add a section to the menu, which I described at the beginning, that leads you back to the car selection screen, right along side the sections for tuning your car and changing the race settings.
In my humble opinion, this small change would make for a great quality of life improvement for all players and thanks to this new suggestion hub, I hope, that it will be added at some point in the future.
Kind regards