Occasionally an existing model needs to be retrained.  This sometimes happens when a model is not detecting all the duplicates or a large amount of new data has been added to Salesforce. 

Notice that an existing model has a version number.  To re-train this model, select it from the list and press OPEN.  

Then select the TRAIN button and CONFIRM to create a new version.

On the next screen, you are asked to evaluate if a pair of records is a duplicate.  You can answer, Yes, No or Not Sure.   The process is the same as training a new model, except the training picks up where the previous version stopped.  

Press the Finish button to complete the re-training.  Note that the a new version of the model will be created.

Good to know:  All the versions of a model are available for selection by a dataset