downgrade from 6.0.1 without root and without losing data?

so a family member asked me to downgrade his samsung galaxy s5 from android 6.0.1 to the previous version.
i've googled it a bit but only found ways where you either lose your data or need root and he won't let me root his phone.
is it still possible and could someone either explain or link me how to do this?
thnx