From PC Gaming Wiki:
Save game data location • Link
| System | Location |
|---|
| Windows | %APPDATA%\StardewValley\Saves\ |
|---|
| Microsoft Store | |
|---|
| macOS (OS X) | $HOME/.config/StardewValley/Saves/ |
|---|
| Linux | $XDG_CONFIG_HOME/StardewValley/Saves/ |
|---|
I used this earlier this year to transfer a save after my old laptop died (fortunately I'd backed up some stuff first) and I built a new PC. I just installed the game and then copied over the old file from a USB into the correct place on the new PC. I'm on Linux, and the first was a hidden file on my home folder, so I just selected the 'show hidden files' option in the drop-down menu to get there. I don't know if the same for mac or windows. Hopefully, that helps.