Leverage on Omni is set per market from the order entry panel, and you can adjust it before opening a position.
In the order entry form, click the leverage button (it shows your current leverage, e.g. 10x).
In the leverage window that opens, set your desired leverage using the slider, or type a value directly.
Click Confirm to apply.
Your new leverage takes effect for that market, and the order form updates to reflect it (you'll see your maximum position size and estimated liquidation price change accordingly.)
Keep in mind that leverage is set per market, so changing it on one market doesn't affect others.