Once you have completed all activities within a module, a link will appear at the bottom of the screen that reads 'Click here to unlock the next module and continue the course'. Simply, click this link to and the content in the next module will be unlocked.
If you cannot see this 'Unlock' link, check that all activities on the course page are ticked. A tick means it has been completed. Revisit and complete any activities which do not have a completion tick next to them. The Unlock link will appear once all activities are ticked as complete.