What Don showed you should be correct.
myPoll = Poll.find_by(post_id: Topic.find_by(id: 7068).posts.first.id)
myPoll.chart_type = 'pie'
myPoll.save
You want to find the Topic data based on the topic ID (7068).
You retrieve the poll data based on the first post ID.
You change the chart type and save.