1. Click the QuickFieldEdit ribbon icon or type QField in the command line.

2. Point the field whose object is to be replaced.
The field could be a part of a text or a block attribute.
If there are several 'field' attributes in a block you can manipulate each of them separately. Try to click ON the field to be manipulated, NOT wherever on the block.
If there are several fields in a single piece of attribute/text the new selected object will be linked to the first (leftmost) field.

3. Point the new object to be linked to the field.
If the new object hasn't got the right property needed for the field (e.g. if you try to link a line to a field showing the area of an object), the field breaks (shows ####). You can use the _QField command on the broken field and linking it to a proper object will 'repair' the field (it will begin showing the property of the new object)