You'll need to separate your plant seedlings. Many container garden plants come in a plastic four or six-pack of cells and sometimes it can be a bit tricky to get the plants out without damaging them. Try turning the whole thing over and then gently squeeze or press the bottom and sides of each cell until the plants slide out. If you have to pull, don’t pull on the plant, which is delicate, grab the soil plug and pull that.
If your lettuce is root bound, gently tear the roots apart, otherwise the roots can grow in a circle and eventually strangle your plant.