You need to query for the workitems using PE API. Assuming workitems are in a queue then,
VWQueueQuery vwQueueQuery = **yourqueue**.createQuery(java.lang.String indexName, java.lang.Object[] firstValues, java.lang.Object[] lastValues, int queryFlags, java.lang.String filter, java.lang.Object[] substitutionVars, int fetchType)
then
while (vwQueueQuery.hasNext()) {
vwStepElement = (VWStepElement) vwQueueQuery.next();
//lock if you want to modify the workitem
vwStepElement.doLock(true);
//once you have vwstepelement, there are different ways to get properties
String[] properties = vwStepElement.getParameterNames();//this will give you all the properties that are exposed for that queue.
//if you want to get a specific property then use
Object specificParameter = vwStepElement.getParameterValue("propName");
//then if you want to set a value
vwStepElement.setParameterValue(parameterName, parameterValue, compareValue);
//finally, if you want save and dispatch to next level
vwStepElement.setSelectedResponse(response);
vwStepElement.doSave(true);
vwStepElement.doDispatch();
}