You can switch networks on the swap page. In order to do so:
Click on the network at the top right to reveal a network selection menu.
If the token you are viewing has liquidity on another network, you are able to switch to that network to swap.
Please note that if the token you are viewing does not have liquidity on a given network, the token will be unavailable to swap on that specific network.
To view a full list of networks supported by Slingshot, click here.