WordNet sense 1 (become less tense, less formal, or less restrained, and assume a friendlier manner):

loosen up

WordNet sense 2 (make less severe or strict):

loosen

WordNet sense 3 (make less active or fast):

slack, slacken, slack up

WordNet sense 4 (make less tight):

unbend

WordNet sense 5 (become loose or looser or less tight):

loose

WordNet sense 6 (become less tense, rest, or take one's ease):

decompress, unwind

WordNet sense 7 (cause to feel relaxed):

unlax, unstrain, make relaxed