In short (and definitely needs a higher level explanation to encompass in full), a net reaction of an organic reduction reaction takes place to get from Digoxin to Digitoxin on the HIGHLY targeted secondary ( 2° ) carbon located on the far right of the Digoxin chain in the picture you linked above.
How to actually go about it and make sure it doesn't reduce any other OH groups in the chain? I'm sorry to say I cannot answer that.
I cannot even begin to imagine the perfect conditions to get that complex Digoxin chain to reduce that very particular OH group and yield that Digitoxin compound.
Also, these compounds are apparently super toxic. I suggest not actually attempting any real-life reactions with these.
