I think what you want to do is perform a Put operation. You can look at the HBase's Put API client docs as well as this blog post titled: Java Example Code using HBase Data Model Operations for some examples, and descriptions of the operations.
To update a row, you will have to issue a sequence of "Delete" and "Put", in a single Mutation, so they are seen as an update externally.
See the Class RowMutations documentation on how to build such a call.