If you want to buy Ethereum with cash, there are a few steps you should take. First, you will need to fund your account. The most common way to do this is by depositing money from your bank account. Other options include debit cards and PayPal. The fees for each method may vary.
After you have funded your Ethereum account, you can sell it. Many centralized exchanges will offer this service. To sell your Ethereum, you need to select the amount that you wish to sell. The exchange will show the fees you'll have to pay. Your cash balance will then be available to send to a bank account.