You can adjust the leverage of your position through the leverage slider in the trading interface.
When you drag the slider to the left, the leverage will be decreased. To drag the slider left, you must have some cash available in your Cash Balance. The cash is then transferred to your Margin Balance, increasing the margin of your position, but decreasing the leverage.
When you drag the slider right, the leverage will be increased. Margin will then be transferred from your Margin Balance to your Cash Balance. Your Cash Balance can be withdrawn and used for new orders, but does not contribute towards the margin of your current positions.
The leverage may change slightly as you accumulate profits and losses. You can always get it back to your desired quantity by adjusting the slider.
Not sure what Cash Balance and Margin Balance means?
Check out this article.