I couldn’t find this in the documentation, so I thought I would post it in case someone else has the same question. If you have an XMLTYPE datatpe column stored as a CLOB, you can modify attributes using the example below.
alter table xwc.xwcmd_xml_data modify lob(mdata_xml.xmldata) (pctversion 1);
Notice the xmldata appended to the column name.
HTH,
Steve