Fun fact: Of Deerling's and Sawsbuck's 4 season-based forms, only one form (spring) were available natively on the Gen 6 games. The Gen 7 games didn't have Seedling/Sawsbuck and neither do Sword and Shield at this moment.
Because Game Freak deprecated the season shifting mechanics of the gen 5 games, There's no way you can change the appearance of these two species on any game made since then. That was 3 Pokemon generations ago.
This means if you haven't done this already, you have to go back all the way to the gen 5 games to catch the other three forms of Deerling and Sawsbuck, transfer them to Bank, and then transfer them to Home to register their data for the National Dex.
This is what I did tonight.
In a new game of Pokemon Black, it took me about 4 hours of speed running through the game and 4 gyms to enter the area were wild Deerling are first found in the wild. I caught 4 of those suckers and in each instance changed my 3DS's date up a month to cycle through all four seasons.
Final steps were transfering them to my Pokemon Ultra Moon game to evolve them to Sawsbuck, bred additional Deerling, and then finally transfering them to Pokemon Home.
Probably the most daunting task of collecting multiple forms of a specific Pokemon if you haven't done it already.