You can go back and view your past seasons and edit your records, change pasture boundaries, add or remove infrastructure, and add or remove hay supplement. Just go into the Ranch dropdown menu, select the past season, and make your changes.
Any changes to pastures, infrastructure, and grazing records you make to a past season will only affect that season. Changes to animals such as weights and IDs will apply to all seasons, however.
You can export your grazing records for past seasons by selecting the season, going to the Pastures tab, and clicking on "Export grazing report".