To make a possessive of an indefinite pronoun, add an apostrophe plus s, just as you would for a noun.
Examples: somebody's child
Personal pronouns, including it, do not have any apostrophes for their possessives.
Incorrect: her's their's your's
Correct: his hers its ours yours theirs whose
If it helps, remember that his takes no apostrophe. In the same way, neither do any of the other forms.
The words it's and who's do exist, but they are contractions. It's means it is or it has; who's means who is or who has.