First there are several relayers and dapps using WETH, each of them can unwrap or wrap as needed.
Also there are two ways to directly interact with the contract, either through MyEtherWallet or through Etherscan. Either one will require contract information from Etherscan.
For reference here are the old and new WETH contract addresses.
RELAY updated to the new WETH address on January 8th 2018.
Old WETH contract address 0x2956356cd2a2bf3202f771f50d3d14a367b48070
New WETH contract address 0xc02aaa39b223fe8d0a0e5c4f27ead9083c756cc2
1. Using MyEtherWallet (with contract information from EtherScan)
Navigate to the page for WETH on Etherscan. Make sure this is the current contract.
Select the Contract tab and `Code`
We will need this in a moment so keep this tab open and open a new tab for the next step.
Open MyEtherWallet / My Crypto
Connect with your choice of wallet go to "Contract"
Interact with the WETH contract
Paste the WETH address under "Contract Address"
Back on Etherscan, copy the "Contract ABI"
Paste that back in Mew under "ABI / JSON Interface"
Select "withdraw" to unwrap WETH
Enter the amount you would like to withdraw. This number will be in Wei. Use a calculator like https://eth-converter.com/ to convert WETH to Wei.
Send the transaction
Access the wallet your WETH is in by selecting MetaMask or Ledger from the options. If using MetaMask, make sure the extension is connected to the correct wallet.
Click "Write," click "Generate Transaction," then click "Yes I am sure! Make transaction."
Submit your transaction via your connected wallet.
Once the transaction is mined your WETH will be unwrapped back to ETH.
2. Use the "Write contract" function directly in EtherScan
Go to the EtherScan page for the WETH contract and select "write contract"
Connect your wallet using the "Connect to Web3" button
This is still a beta feature.
In the section labeled "withdraw" enter the amount of WETH to unwrap. Enter the amount you would like to withdraw. This number will be in Wei. Use a calculator like https://eth-converter.com/ to convert WETH to Wei.
Press the "Write" button to send the transaction to your connected wallet.
Confirm the transaction to send to the Ethereum blockchain.
Once the transaction is confirmed your WETH will be unwrapped.