I'm not a professional in this, but if you're playing on your phone. I think it's impossible to transfer it to another, or if you're on your computer and want to transfer it to your phone, vice versa.
But if you are playing on your computer and want to transfer it to another. Maybe you can connect both computers, take the save file, and transfer it.
for mobile you can install ZArchive and navigate to "storage/primary/Android/data/com.r3nn.ld/files/saves". you could copy your save files from there to an easily accesible folder and upload them to something like a google drive to then download them to your new device, and then once again use ZArchive to copy them from your download location to the save location.
if you're on a computer you can just upload them directly to the Google Drive (or whatever you use) from (allegedly) somewhere in AppData>Roaming>RenPy on windows, not sure exactly about mac.
