Thanks for letting me know.
You can manually delete the old save.
If your on windows its located:
[Main Drive, usualy C]\Users\[UserName]\AppData\LocalLow\TexelHeim\MythGotchi\MythGSave.MythS
On Android its usally easiest to uninstall the game and re install it. But the save is located here:
/storage/emulated/0/Android/data/com.TexelHeim.MythGotchi/files
You might need a pc to get access to the path manually
On Linux you should be able to find the save here:
~/.config/unity3d/TexelHeim/MythGotchi