If you already have an opened position and you would like to modify your position, you are in the right place.
1) Start on the Trade page and on the right hand side you will see a panel to open a Long or Short position. In this example, I have a Long position open worth 500 USD.
If I wanted to extend this Long position, I can start on Long tab and spend the available buying power to further extend that position.
If I wanted to reduce this Long position and/or open a Short position instead, I can start on the Short tab and spend the available buying power to first decrease the Long position to 0 and then convert to opening the Short position, as shown below.
2) Once you're satisfied with the input amount, click on "Open position" and confirm the transaction from your wallet. Wait for the interface to update and you will see your position show up in the lower panel of the screen.
In my case, I successfully converted my Long position to a Short position.
3) Good looks! You have successfully modified your position 🎉