As soon as the workflow is executed, FireStart forces a new version of the process model when changes are made. The version of the workflow is displayed directly under the name of the workflow. The version is always taken from the process model, which means when the process model is executed in version 1.5 then the workflow has the same version. The version can't be changed after the execution. That way you always know which workflow is the current workflow because the latest process model has the same version as the workflow.
But there is an exception that could lead to misunderstanding, which is the linked processes. These processes will not be executed in the linked version of the model but in the latest deployed version.
You can go back to every previous version of a model. For that, you have to click Version Overview in tab Main under the section Model Management.
With double-clicking, the model you would like to use, the model in this version will now be displayed when clicking Open.
For more information about model versions please have a look at the topic Model Version and Updates.