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.