If your Suica card isn't set as your default payment card:
On your iPhone X or later, double-click the side button. On your iPhone 8 or earlier, hold the top of your phone near the contactless reader without resting your finger on Touch ID. When your default card appears, tap it, then tap the one that you want to use. To pay, use Face ID or rest your finger on Touch ID and hold the top of your phone within a few centimeters of the contactless reader until you see Done and a checkmark on the display.
On your Apple Watch, double-click the side button to access your cards in Apple Pay, then swipe to select your Suica card. To pay, hold the display of your Apple Watch within a few centimeters of the contactless reader.