To set a Leverage simply move its value on the slider. Primex allows for custom leverage in increments of one-tenth.
When changing the Leverage, the order size stays the same while your Deposit Size changes based on the selected leverage value according to the formula:
Order Size = Deposit Size x Leverage
Besides your Deposit Size, the selected Leverage affects the Liquidation Price at which your position will be automatically closed to prevent further losses. The bigger leverage you choose, the more risk there is, because the distance from the current market price to the price against your anticipated market movement shrinks (as the Liquidation Price itself changes).