As a first step, select Design Mode from the Modes menu. Click on one of the outer segments of the helix, and mutate it to lysine (LYS or K). Do a quick shake. At this point, you may get a popup saying "You've earned a new tool!". (The Foldit central team really, really likes exclamation points (also known as bangs!)!!!!)
If you don't get any bang, try mutating another segment to lysine, and shake again.
Eventually, the Mutate button on the Actions menu should become available. A popup tells you when, mutate will be greyed out until then. Click on the Mutate button, and let it run until the score reaches the required value. Stop the mutate tool, and the puzzle should be complete.
Take a look at the front helix. The outer sidechains (about six of them) have changed to blue hydrophilic amino acids. The inner sidechains (again, six or so) have remained orange, but several of them have changed to a different amino acid.
Technical stuff: the exact details of what the mutate button does are not known to Foldit players. Many Foldit recipes do mutations. Some try every amino acid at every segment (usually in a random order), also known as the "brute force" approach. Other mutate recipes try to replace amino acids based on similarity, such as replacing hydrophilics with hydrophilics.