From the picture, this weed appears to be redstem filaree. It is a winter annual or biennial that spreads by seed. Stems are a few inches to up to a footlong, is spreading and grows from a basal rosette. Leaves and stems are hairy. Flowers are pink or purple.
Here is a link to more information: https://www.colorado.gov/pacific/agconservation/redstem-filaree. If weed is not widespread you may wish to remove the weed by hand. The most common herbicidal control for broadleaf weeds in the lawn is 2-4, D. Please read and follow all label directions for safe use and application.
Here is a link to more information on broadleaf weed control in the lawn: https://planttalk.colostate.edu/topics/lawns/1525-controlling-broadleaf-weeds-lawns/