You can snip any unwanted plants off near the ground or you can carefully transplant some of them. Plants are usually fairly delicate to transplant prior to getting their first true leaves, but with care they should transplant o.k. Keep in mind where their eventual home will be, after all you probably don't need 50 or 100 lettuce plants.
With lettuce seeds, the gardener just needs to be careful not to place too many seeds to close together. I wouldn't plant more than 3-4 seeds per cell in a tray. Beets are a little different. They are like Swiss Chard, in that each seed is really a seed cluster and not a single seed. So don't plant more than one seed every few inches or per cell, then be prepared to thin the plants, either by snipping or lifting and transplanting them.
Eclectic gardening style, drawing from 45 years of interest and experience. Mostly plant in raised beds and containers primarily using intensive gardening techniques.