To top up a web3 wallet using the Uniswap fiat onramp (https://app.uniswap.org/), you will need to follow these steps:

1. Connect your web3 wallet (such as MetaMask) to the Polygon network

2. Click on the dropdown on the Uniswap website and click on the "Buy crypto" button

1. Select the currency you want to buy on the Polygon network (e.g. USDC or MATIC)

1. Enter the amount you want to buy and click "Next"
2. Verify your transaction and confirm the details, including the exchange rate and fees
3. Send the payment to the provided bank account or use your credit card to complete the transaction
4. Once the payment is confirmed, the tokens will be sent to your web3 wallet.