You can always change the description, date and method of assessment of an assignment through Edit Assignment. The type of assignment and how students have to complete an assignment, however, can only be altered as long as no student has submitted their work. The settings Assignment Type and Submission Type play a role.

• Navigate to Activities in the navbar of your course.
• Click Assignments.
• Click the fold-out arrow next to the desired assignment and then click Edit Assignment.

If students have not yet handed anything in, you can use Submission & Completion to alter whether it is an Individual assignment or a Group assignment. 
You can also alter the Submission type: as a file (File submission), as a comment (Text submission), hard copy (On paper submission), or as a performed task (Observed in person).
You can choose how many assignments students can submit (Unlimited or One File), and what type of file they can hand in (Allowable File Extensions).
You can also choose which submissions will be saved (All submissions are kept, Only one submission is kept, or Only the most recent submission is kept).