Change Unfitness to
This is a very important edit box because it allows you to manually set the unfitness of a design. If you make changes to a design and you want them to have a chance to breed into the population, you should set the unfitness to a low value so that the design has more offspring. It is important to realize that if this design has a very high unfitness to begin with, the characteristics that you like in it may well be being selected out of the population by the agents. In order to have a more lasting effect on future generations, it may be necessary to edit the agent preferences. However, to give evolution a single sharp nudge, nothing beats modifying an individual and giving it a low unfitness.
You may find it useful also to introduce several copies of this design into the population with the "Make _ Copies" edit box to further enhance its influence.