Use canned compressed air and blow off the dust and dirt on the lenses.
A fine make up brush will clean the rest and then use a camera lens cloth.
DO NOT USE 40 grit sandpaper or a shirt tail with a screwdriver or pocket knife to get in around the edges and gouge out a bit of glass..
You would not believe what "little boys with vise grips" can do to a scope and adjustments as well.
An achromat lens is made of two layers of glass glued together. One of flint glass and one of crown glass.
Glass will scratch very easily and if you can feel the scratch with a finger nail the lens is pooched.