This article is specific to schools accessing the GTT as part of the National Institute of Teaching's ECF Programme.
You certainly can! If you have wrongly paired up a Mentor with an ECT, or if the pairing needs to change for whatever reason, you can do so as an Induction Tutor in your "Early Career Framework" page.
In the table of teachers, under the "Mentor" column, you will see there's now an orange "X" to allow you to unpair the two. Simply click this, and you will be able to reassign the ECT correctly.