Is it obligatory for someone who replaced a friend's computer part with a worse one, and paid an undue amount six years ago, to return the money at its current value or its old price, and should he buy the replaced part at its current price or its old one?

You must repent and return the part you took to its owner, guaranteeing compensation for its deficiency, unless the owner forgives you. You must also return the equivalent rent for that part for the period it was with you, refund the repair cost you received, and return the value of the amount you took if the currency's value has changed. You must promptly return all of that immediately. You do not have the right to sell his device and buy the part for yourself. You can borrow the device from its owner, install the original part in it, take the other part, and send him the amount you took from him, the value of the original part's deficiency, and its equivalent rent.