Most assignments need to be completed by a specific deadline. This applies for Peer-to-Peer and Quiz assignments in particular. Please notice that you can not edit those entries after you submitted them. If you missed the assignment deadlines because you joined late it is not possible to repeat the assignment. In a Peer-to-Peer assignment a blank text will be submitted. Don't worry though! A missed assignment will not let you miss the 80% treshold for your Statement of Participation.
Journal assignments are in most cases not bound to a specific deadline and can be edited afterwards. To edit your journal assignment, click on "Journal" in the navigation bar in a specific course.
Under "My Journal" you have an overview of your journal entries. If you choose a specific entry you have the option to edit or delete your entry. Please keep in mind, that it is not appropriate to change your journal entry if someone has already commented on it. In this case it is better to answer the comment.