Hello David,
How do I reset the attempts for a single question
I don't think you can reset a single question in a quiz.
retry this question without deleting the student's work for the rest of the quiz
Under "Edit settings" then under "Question behaviour", set "Each attempt builds on the last" to "Yes". You can do this even after the quiz has started. Note that once "Each attempt builds on the last" is set, the new attempts will contain the results of the previous attempt, for any student who is allowed to reattempt.
I want to give a single student a change to retry
Under "User overrides", click "Add user override", then set "Override user" to the student's name and "Attempts allowed" to 7.