iPhone or iPad
Go to your phone general Settings > Wallet & Apple Pay
Tap the card that you want to remove, then scroll down and tap Remove this card.
You can also open the Wallet app 
Tap the card you want to remove, tap the three dots, then scroll down and tap 'Remove This Card'
Apple Watch
On the iPhone, open the Apple Watch app, and go to the My Watch tab
Scroll down, tap Wallet & Apple Pay, tap the card, then scroll down and tap Remove This Card
Or tap Wallet on the Home screen of your Apple Watch, tap a card, press it firmly, and tap Remove Card