Can you harvest your lettuce and then will it grow another harvest?
You can pick off the outer leaves first. The plant will grow more from its basal growing point.
It is actually recommended to do as @JohnLim just mentioned if need to be harvested gradually, not all leaves at once (which is another option).
Also, we have learned that pods that have been harvested gradually last longer, produce more leaves and there is less leggy growth present (plants stay more compact).
Though, please keep in mind that lettuce is an annual plant and the pod does not last longer than ca two months (really dependent on harvesting style and temperature).