Smapi automatically creates backup saves for you juuuuust in case anything goes wrong. (Nothing usually goes wrong.) You can find these saves in Stardew Valley\save-backups. To use them, extract the file named "SaveGameInfo" and "Farmname_Date" into your main Stardew Valley saves folder. This folder is located at C:\Users\User\AppData\Roaming\StardewValley\Saves.
These instructions are for Windows and may be different for Mac or Linux.