The week that the planner put an assignment into isn’t ideal for me. How can I change it?

No problem.

Just go to your All Checklists page and use the "Re-Assign to Week" drop-down lists to move it to your preferred week.

Or you could also go to the Add/Re-schedule page and be sure the deadline is in the week you want, and then check the "Fixed Date" box so that it is guaranteed to be scheduled in that week from now on.

Be sure to save your changes and update the planner.