If you go to settings > Security and User Permissions> user management > users, you will find a list of all of your current users. 

Click the blue ''setup'' button next to the user in question to change their permissions. 

Simply tick a permission to award this functionality to this user or untick a permission to revoke access to a given feature. Just be sure to press save at the bottom of the page to save your changes. 

Alternatively, if you click the edit icon next to a user, you can add this user to a new group of permissions. 

Once again, just be sure to press save when you have completed your desired changes. 

Once a user refreshes their screen or logs out and back in to the system, they will be able to see the changes in their access level.