Yellowing of needles on conifers can be caused by needlecast diseases, mite damage, nutrient deficiencies or water stress. Click on the link below to read more of the details about each.
https://www.canr.msu.edu/news/yellowing_interior_needles_in_conifers

I suggest you contact a certified arborist who can come out and take a closer look at the tree to determine the likely cause and recommend a treatment strategy if needed.  You can find a certified arborist in your area through the web site below. There will be a charge for the service call so you may want to call and determine which one you prefer to work with.  https://www.treesaregood.org/findanarborist
Hope that helps.