Okay, so in that situation the one-to-many relationship is created by adding a 'Change request' reference field on the 'Change task' table. You can see the field if you open a change task record. You can also identify the field in your screen shot filter above the 'Change tasks' related list where it says 'Change request = '. This part of your filter isn't editable because it is the relationship field that ties those 2 tables together. The 'Change Tasks' related list is created at the same time as the 'Change request' field to complete the 'Many' side of the relationship.
If you want a new 'One-to-many' relationship between 2 tables you just need to create a new Reference field on one table and then add the related list on the other.
