You can change the contract Leverage after its creation by clicking the Margin button in the position line:
Additionally, you can select the
button on the banner at the top in order to change leverage.
You can either use the leverage slider or type in the amount of leverage you would like to update your current and any future position to. 
Please note that you can see the changes to your Buying Power, Liquidation Price, and Risk of Liquidation as you increase or decrease the position’s leverage amount.