In order to add more OXT to an existing Orchid account on Ethereum, first visit https://account.orchid.com and then perform the following:
Ensure your wallet app is connected to Ethereum
Connect it to the dApp if and when prompted
Select the "Add" button
Use the slider to determine how many OXT you're adding to your account based on your preferred account efficiency
Choose the account size
Select "Add Funds"
If prompted, approve Orchid's smart contract to allocate OXT on your behalf, and confirm the transaction
When that transaction is complete, confirm the transaction to allocate your OXT to Orchid's nanopayments contract
The Orchid dApp interface will inform you when your transaction is complete! Now you have a topped up Orchid account you can continue using.