From
The Elements of Style:
Form the possessive singular of nouns by adding 's.
Follow this rule whatever the final consonant. Thus write,
Charles's friend
Burns's poems
the witch's malice
Exceptions are the possessives of ancient proper names ending in -es
and -is, the possessive Jesus', and such forms as for
conscience' sake, for righteousness' sake. But such forms as Moses'
Laws, Isis' temple are commonly
replaced by
the laws of Moses
the temple of Isis
The pronominal possessives hers, its, theirs, yours, and ours
have no apostrophe. Indefinite pronouns, however, use the apostrophe to show
possession.
one's rights
somebody else's umbrella