Basically you'll need to remove the period end date.
Depending on if you want to do this for an individual or all participants, follow the "how to" below.
1. Individual participant
Go to their purchase, "Edit purchase" and edit the "When does the current access period end" date, and click save.
2. All participants
Go to the Product > Purchases.
Check the checkbox next to "Name", scroll down and select "Set subscription period end date", leave it blank and click save.