Choose the Season that you want from the downloads page. Only have ONE Season enabled at a time. You can manually switch seasons via your mod manager at any time, but Seasons do not change automagically. You don't need to start a new game when you remove a Seasons file, because the plugins are empty. When you first go back into the game after removing a season you may get a warning about the missing file, but it's safe to continue.

Automatic Season Changes are a technical impossibility.

Seriously, it is impossible. We've asked several prominent F4 scripting experts. Not even F4SE can do it. Please don't ask for it.

Sometimes you may get dead trees that are replaced by prewar trees, you can use the console to delete any unwanted replaced trees that aren't covered by the settlement scrapping system.

select the tree in the console, enter:
disable

if the tree disappears it's safe to then enter:
markfordelete

if something ELSE disappears or you can't tell what disappeared, enter:
enable

COMBINED OBJECTS: I don't recommend disabling combined objects for Seasons. It kills performance, and Seasons is not designed around it.