How to use JDBC to update LargeObject

I am using 8.1 Postgre JDBC. I want to know how can I use JDBC to
update the contents of a LargeObject. For example, say I have created
a LO for a file with 1 Mb of data. Later on, file contents have
changed and are now just 100 kb and I want to update the LO. When I
try to write to a large object using the method largeObject.write(byte
[], off, len), it seems to be updating the first 100 kb data, not
really replacing the existing 100 MB with the new data. pg_largeobject
table still shows the same number of rows. Any idea how can I update
the contents of a large object.