Hi There,
Can we edit credit card fields place holder web payment sdk as we can do that with older api:Square Connect API Documentation
The Web Payments SDK lets you customize the style of the card entry and gift card entry forms. You can view the supported stylesheet classes and properties in the CardClassSelectors object API reference.
The five visual elements of the Card payment method are represented as selectors that you specify in a card option styling object. You can set colors, fonts, and border options. 
Can i change field placeholder?
Unfortunately, the field isn’t customizable the color of the placeholder is.