Think you pretty much missed the boat. Depending on what lettuce you have, it may grow. You might want to try some chard.

Here in upstate SC, my main heat tolerant summer greens are malabar spinach and leaf amaranth. Spring planted Swiss chard will keep going all summer (and all winter under a cold frame). Lettuce is a challenge during the heat of summer, first to get germinated, and then it bolts before it gets much size to it.