PS4 Savefile Backup:
Go to PS4 Settings > Application Saved Data Management > Copy to USB or PS+ Cloud (if you use PS+ Cloud make sure you disable auto-uploads for Sekiro or else it will overwrite your backup save automatically). If you like to keep multiple backups from different points in the game, you can copy the PS4 folder from USB flash drive to a computer, this way you can have unlimited save game backups.
Xbox One Savefile Backup: Exit the Game (exiting the game overwrites cloud save!) > as soon as achievement pops, press Home-Button > scroll down to Sekiro Shadows Die Twice > press Options > Manage Game > Saved Data > click Gamertag > Delete Saved Data > Delete from Console > if it still shows the save, delete it again until it’s gone > press Home-Button and restart the game > it will now sync / download your save from cloud! If in doubt about any step please test it on a secondary dummy account first where you don’t risk losing any progress.
After each ending you will need to restore the backup save (copy it from USB / PS+ back to PS4)