The turn in a sonnet refers to a shift in focus or thought. The turn is in fact a metaphor, as the speaker is metaphorically "turning" from one thing to another. In this excellent sonnet which acts as a declaration of how important the love of the beloved is to the speaker, and how it makes him the envy of all men, the turn comes in the final rhyming couplet. This can easily be identified because the tone and the topic distinctly changes as the speaker begins to think about what would happen if he were to lose the love of his beloved:
Wretched in this alone, that thou mayst take
All this away and me most wretched make.
This is in stark contrast to his joy and pride as expressed in prior lines, which communicate the glory and status he feels he receives because of his love for his beloved. The repetition of the word "wretched" emphasises the perilous position he is in. He has so much invested in this relationship that were it to end, he would be truly "wretched."