lessen or to try to lessen the seriousness or extent of
The circumstances extenuate the crime.
source: WordNet 3.0
make less severe or harsh
Mitigating circumstances.
source: WordNet 3.0
To make less severe, intense, harsh, rigorous, painful, etc.; to soften; to meliorate; to alleviate; to diminish; to lessen; as, to mitigate heat or cold; to mitigate grief.