There are three ways to do this.
The easiest way to unwrap would be through the 0x Portal. Simply click unwrap and confirm the transaction. This will also work for deprecated WETH.
The next easiest would be to go through another relayer.
Then, even if every relayer and the 0x portal happened to go down all at the same time, there is still a way to unwrap your WETH by interacting directly with the WETH contract. Here's how:
Find WETH on Etherscan
Navigate to the page for WETH on Etherscan.
Click Contract Source
We will need this in a moment so keep this window open.
Open MEW / My Crypto and go to "Contracts"
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"
Unwrap your WETH
Select the "Withdraw" option from the drop-down
Enter the amount you would like to withdraw. This number will be in Wei. Use a calculator like https://etherconverter.online/ to know what to enter in that field.
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 on MetaMask or Ledger.
Once the transaction completes your WETH will be unwrapped back to ETH.