You just have to download the new build and unpack it. You can feel free to remove the past build, saves are stored in a separate folder
Saves should be compatible with the newest build, unless for various development-related reasons the amount of changes in the build would essentially break that compatibility. Whenever something like this happens, we inform players about it on the dev journal
As for timeline, the game takes place over the course of the single year. It starts in early April and ends around late March (excluding epilogues, that is)