Yukka Tree

[Cellerton]

How can I trim my Yukka Tree

[alisonkal]

You can't trim the leaves of a Yucca, it will look horrible and the ends of the leaves will go brown. You didn't give a lot of detail in your question, is it too high ? too wide ?
If it is too high there is a way but will take a long time to recover. You have to saw the top of the tree, removing all green growth, then you have to seal the stump with wax. In time your yucca will produce new shoots from the side of the trunk.