1. Open steam console by typing steam://open/console in your search engine (or with Win+R on Windows) while Steam is running.
2. Go to the steamdb manifests page for your operating system (Windows / Linux / Mac) and find the manifestID of the update you want to downpatch to. You can check the date an update came out via the Game Updates page.
3. In steam console, type download_depot 322330 32233X ___ but replace X with 1 for Windows, 2 for Linux, or 3 for Mac, and replace __ with the manifestID you found in step 2.
4. Wait for the depot to download. This will take a while, much longer than a regular game download. You're free to launch steam games during this time.
5. Eventually you should see "Depot download complete :" in steam console followed by the path to the depot. Navigate to the depot_322331 folder in that path and copy it.
6. Navigate to your "Don't Starve Together" folder. You can do this by right-clicking DST in steam library, then selecting Manage > Browse Local Files.
7. Step back 1 folder to "common" and rename your "Don't Starve Together" folder to anything else, such as "Don't Starve Together current version".
8. Paste the depot_32233X folder into "common" and rename it to "Don't Starve Together".
9. If the update you're downpatching to is on or before August 12th, 2021, you'll need to launch the game in 32-bit mode. You can do this by right-clicking DST in steam library, then under Launch Options, click the "Selected Launch Option" dropdown menu and select "Play Don't Starve Together (32 bit").

And from there you're set! If you want to switch back to current patch, simply switch the names of the folders again. For example, you could rename "Don't Starve Together" to "Don't Starve Together downpatched version" and rename "Don't Starve Together current version" to "Don't Starve Together"

Big thank you to Carrion for explaining these steps! c: