Drag and drop from the node (from the green number in the top right of the node) to the required word in the left panel (the tectogrammatical tree). The word will appear in the middle part of the node.
Notes:
It will not work the other way round (from the tree to the node)
To change the reference, simply drag and drop to another word, the original word will be replaced
To delete the link, Select it and press Delete key orbutton on the toolbar.