If the yellowing is ooccruing on the inside of the plant, and the growing tips remain green, do not orry; Old leaves (in this cae, the needles) die off after a while. Typically you get one flush of yellowing each year when the old needles are rejected by the tree.
The tree does not look too bad. Do you know what substrate is in the pot? If it is this all over you need to ensure you give loads of water each time you water. So if you have a showerhead for your watering can, use that, and make sure you cover the whole pot. Then water till it runs out at the bottom. And indeed, then wait for the pot to start to dry out before you do this again. How much / often have you given fertiliser?