You can delete local models from the Settings View in the Runway app.
You can access the Settings View by clicking the circular button with you user profile profile picture / username on the bottom left corner of the app. Inside the Settings View, you can see your models listed under "Local Models" on the right side. You can delete any of the models listed there to free space on your computer. There is no harm in deleting these models as they can always be downloaded again later.