Ideas in the Canvas are ordered by the date they were added, and will be reflected in the Priority List in that same order.
To update this, click into your project Workspace. In the Priority List that appears to the left, drag-and-drop ideas via the grid icon that appears to reprioritize them.
This will then be reflected on the Canvas. Keep in mind that the Priority List shows only ideas in testing by default; you can use the filter to view those you've validated or invalidated as well.
Keeping your Priority List updated in terms of the most critical ideas will help keep your team on track in testing your riskiest hypotheses!