Martina P wrote:
You can't. If you want to upgrade, you need a credit card.
This is incorrect. According to Upwork, you can use the balance to pay:
Can I use my earnings to pay for a Freelancer Plus plan?
If you have a sufficient balance available from Upwork earnings, we will use that to pay for your membership. If your balance isn't sufficient, we'll charge your chosen payment method."
You need to upload a payment method but you can use Upwork balance to pay.