You can get the sequence by using GetSequence on a dataset or a dataset item, and the list of sequence items via Items:

var referencedSeriesItems = dataset.GetSequence(DicomTag.ReferencedSeriesSequence).Items;

Then you have to may iterate over the items and find the one you need, and use the same to get sub-sequences. Provided you need the first item:

var referencedInstanceItems = referencedSeriesItems[0].GetSequence(DicomTag.ReferencedInstanceSequence).Items;

Same here - find the needed item(s), and here you can read or write tags:

var sopClassUid = referencedInstanceItems[0].GetString(DicomTag.ReferencedSOPClassUID);
referencedInstanceItems[0].AddOrUpdate(DicomTag.ReferencedSOPInstanceUID, "1.2.3.4");

etc., you get the gist.
Note that this is out of my head, so there may be typos.