If the user who was part of the approval process is unavailable/ left the company, process instances will not be able to complete until you modify the assigned user/approver of the task.
Changing the approver of a workflow instance which is already started will not be possible to do via UI. But you can do this by manipulating the database. Please note that direct modifications to the K2 database is not supported and it would negatively affect auditing/reporting for the process instance.
Below are workarounds that will not involve direct database modifications:
1. Use the Redirect option to redirect the tasks to another user.
2. Set up an Out Of Office for the current user and add a 'Forward To' user, then all the tasks will be available in the forwarded/new user's worklist.