Set multiplexer inputs to a logic 1 state
You understand that to set a logic low on an input, you need to pull the pin to GND. To set the pin to a logic high level, just do the opposite and connect the pin to VCC. You can do this, either, directly or using a pull-up resistor.

Using a resistor, would make it easier to re-work the board, if the design requirements change after the board has been manufactured.