You can disable trailing stoploss on a position by two ways:
Bottom Trade Panel: By tapping on your active SL, you can trigger the SL order form. Therein, you can disable the Trailing SL on by deselecting it in the order form
Option Chart: When you have an active SL, that is visible on the option chart. You can tap on the SL pill on chart, within the further setting on SL, you should be remove the Trailing SL by tapping on "Remove Trail"