Strategic self-deprecation will only get you so far, Neil. It's perfectly understandable to not "get" techie stuff right away. Those more educated in Unity (not me) constantly struggle to comprehend the deep sense of cumulative aggravation this instills in amateurs like us, let alone sympathise with us when the constant onslaught of tiny, undeserved irritations starts to fray our tempers or when the never ending wear and tear induces self-violence, makes us shake our fists, swear loudly, strangle kittens or pitch frogs.
Not sure if this is the source of your woes, but every tree object (see the little arrow on the right of its icon?) can be double-click opened to reveal its insides: Geometry file, different texture types, material, possibly billboards etc for you to tamper with.
What's really annoying with trees is that the texture is not the same as the material (which contains layers of textures and can be set to many different presets). The material you need to fine-tune should be inside a folder called materials. Clicking on the tree icon in the inspector won't help you, you first need to open up its innards and open these in the inspector to get to the menu you seek.
Sorry if you knew all of this basic stuff already. I'm with you, pal. For me, a tree is a tree and Unity, Maya, Bender, Speedtree and all those others can keep all their inner workings to themselves, plant their tree just like that and add understandable settings in one succinct submenu. But no, they are clearly of heinous character and hate normal people, so they're playing hide and seek with us.