By law, you have the right to trim tree branches that extend into your property line. However, it is good practice to bring up the issue with your neighbor first. Give them a chance to take care of the trimming themselves, and/or offer to do it if they prefer.
Keep in mind that California has very strict laws regarding tree trimming. Under section 3346 of the California Civil Code, a person may not cut or remove any part of a tree that does not overhang their property. If you cause the tree to die or cause so much damage that its value is destroyed, you may be held financially responsible. Furthermore, certain trees such as the native oak, walnut, sycamore and willow are protected by Diamond Bar ordinance, even if they are on private property. If the pruning is extensive, you need to first obtain a permit. Refer to Chapter 22.38 of Diamond Bar Municipal Code for more information on tree preservation and protection.