To be a little more precise, use nlapiLoadRecord to fetch the record from the database. Set the fields as needed. Use nlapiSubmitRecord to save it.
Untested code off the top of my head:
var record = nlapiLoadRecord('record_type_goes_here', internal_id_of_record_goes_here);
record.setFieldValue('field_internal_id_goes_here', 'value to set goes here);
nlapiSubmitRecord(record);
Fill in the appropriate values where needed.