As of the moment, there isn't an option to open an already closed time activity transaction.
In the meantime, you can create a new single time activity and then copy it to your customer's invoice.
Let me show you how:
Once completed, you go to your customer's Transaction List and then copy the billable time activity to your customer's invoice. Just click the Create invoice button in your Time Charge transaction.
In addition, here's an article you can read to learn more about time activities: How to Enter Billable Time by Job?
Lastly, clicking the Gear icon in your Time Activity let you customize the details shown on your transaction.
Let me know in the comment section down below if you have any other questions. I'm always around ready to help.