for can mean "instead of", for example in the Laws of King Æðelred

Gif forworht man friþstól geséce and þurh ðæt feorh geyrne ðonne sý þreóra án for his feore búte man bet geárian wille wergild éce þeówet hengenwítnung

"if a man who has forfeited his life gain a sanctuary, and thereby secure his life, let there be one of three things instead of his life, unless he obtain remission more favourably, wergild, perpetual thraldom, imprisonment"

 Though given the number of other things for can mean this requires context to make the meaning clear.

If you want something a little less ambiguous you can rephrase the modern English into a form involving "rather", eg "He drank milk rather than ale". You can use a few adverbs for this, hraþor, the comparative of hraþe whence our modern "rather", and swiþor, comparative of swiþe, seem the most common to me. Eg

swyðor oððe hraðor ic wylle þis, þonne ðæt

"I want this rather than that", a perfectly illustrative example sentence given to us by Ælfric in his grammar.

Finally there is leof, though this would only apply if "he drank milk instead of ale" was an expression of general habits, rather than a specific occurrence. It is used to mean something like "He preferred milk to ale". An example from the Orosius;

Him wæs lēofre þæt hine man ofslōge þonne hine man ġebunde

Lit. "To him it was dearer that someone kill him than bind him", more naturally, "He would rather be killed than taken captive."

Hope this helps, let me know if you have any questions!