To make a swap on Uniswap, you must have some ether in your wallet, since all transactions on the blockchain incur gas fees. 

Buying MXNB using USDT. 

1. First, go to Uniswap.
2. You will need to connect your chosen Web3 wallet so that Uniswap can read the balance you have of different tokens, including ether (ETH), for the payment of gas fees.
3. Enter the token you want to sell and select MXNB as the token you want to buy.
4. Once you review the transaction details and select "Swap", you will receive a notification in your Web3 Wallet to approve the transaction. 
5. Depending on the wallet you use, you might then receive a request to sign the transaction. This is an additional step to check that the transaction details are correct.
6. Finally, you will need to confirm the transaction from your Web3 wallet. 
7. That's it! You should see your MXNB purchase reflected in the balance of the Web3 wallet you are using.

To sell MXNB in exchange for another token, the process is the same. The only thing that changes is the token you select to purchase. To sell MXNB, remember that you must have an MXNB balance in your Web3 wallet.