ORDS.DEFINE_HANDLER - you can use this to update a handler
Otherwise, yes you have ORDS.DEFINE_MODULE which rebuilds the entire module for making changes to any of its particulars.
When using SQL developer I recommend to capture the ORDS.DEFINE_MODULE and ORDS.DEFINE_HANDLER statements in a script, which you can check-in into version control. Just capture the statements from the statement log after you have defined the module, template or handler from the REST dataservices node.
I have many similar REST templates, so I usually copy a previously created file with the DEFINE_TEMPLATE and DEFINE_HANDLER statements and just edit the name, the SQL statement and (if present) the DEFINE_PARAMETER statements. The SQL statement I usually capture from the statement log, so I do not have to do the escaping of quotes in the query (if present) myself.