Go to Call log tab in To-do List and double click on the call log you need to edit.
Under Link to matter, click X to delete the matter it is currently linked to.
A pop up will show up that this related call and tasks in it will be updated, just click OK
and enter the correct matter.
There will be reminder that the current call log and associated tasks in it will be moved to the new matter then click Save.
If the call log has a time associated with it, you will get this error below
So you will need to check the call log or task in it in Financial > Time tab in the matter and delete them from there
and re-enter the correct matter again after.