Changes only the fields you send. Everything else stays as it is.

Any status. A completed task can be edited or reopened. Moving a
loan task into Completed adds a note "

Clearing a field. Send null to clear reminderAt, priority or
details, or to detach the task from its loan (loanId: null). The
other fields cannot be cleared: null for subject, dueAt or
status is accepted only when the task already has none, which a task
made in LendingWise can lack. A credential limited to some branches
cannot detach a task from its loan (422).

Assignees. Send assignees to replace the whole list; leave it out
to keep it. At least one id is required, unless you are sending back an
empty list the read gave you (a task assigned only to a borrower, say).
Only ids you add are checked.

Moving the reminder. A new reminderAt means the reminder will be
sent again, even if the old one already went out.

Sending back what you read. You can send a task exactly as
GET /api/v4/tasks/{id} returned it: nothing changes, and nothing in it
is checked again, so an assignee who has since left your company does
not make the request fail. Any field equal to what the read shows is
treated as unchanged. The read-only fields (id, createdDate,
createdBy, reminderSent, isAutomationTask) are accepted when
unchanged and refused when not.

No email is sent on an update, even if you turn sendEmailNotification on.

404 and 409 mean what they mean on GET /api/v4/tasks/{id}; 409
also answers a move onto an archived loan.