To edit the XML in an OME-TIFF file you can use tiffcomment, one of the Bio-Formats tools.
To use the built in editor run:
tiffcomment -edit sample.ome.tif
To extract or view the XML run:
tiffcomment sample.ome.tif
To inject replacement XML into a file run:
tiffcomment -set 'newmetadata.xml' sample.ome.tif