Motion is passed at board meeting. Later someone wants to change it.

[Guest Felicia Thomas]

At board meeting, a decision was made, voted on, and passed. Later, a non-present board member, by email, wants to consider a change. How must we proceed?

[Tim Wynn]

At board meeting, a decision was made, voted on, and passed. Later, a non-present board member, by email, wants to consider a change. How must we proceed?

At the next board meeting or meeting of the parent body the motion can be amended, if it's not to late. See RONR (11th ed.), Section 35.

[Guest Edgar]

How must we proceed?

But note that there's nothing that the board must do.

[Guest Felicia Thomas]

Thank you for responding so quickly.

[Gary Novosielski]

At board meeting, a decision was made, voted on, and passed. Later, a non-present board member, by email, wants to consider a change. How must we proceed?

Must? There is nothing you must do. And there's nothing you may do by e-mail.

The member, if he shows up for the next meeting, can move to Amend Something Previously Adopted, which requires a majority vote with previous notice (which it sounds like you won't have). Without notice, it requires a 2/3 vote, or a majority of the entire membership.

That's presuming the original motion hasn't already been carried out by then.