You can do this easily if you have sufficient balance in your wallet.
Just follow the steps given below:
Open the Admin Panel of your portal
On the Users page, click the highlighted search box
Enter the username of the account you want to the said action for and hit Enter
The searched username's entries will come to the top
Spot the concerned account and in its row, click on the Extend Days button
You will be taken to this page where you can extend the subscription period
Your plan is selected by default, but in case you have multiple plans, please confirm once and choose the desired one
Click on the blue Update button and wait for the page to reload
3 messages will popup as shown below, this means the subscription duration of that USer Account has been extended.