This article is for reflector type telescopes as I have and am only comfortable servicing my Dobsonian.
With lens's if it is only dust on the lens surface itself try blowing lightly on the lens, if no joy a camera blower brush is the next step blowing across the face of the lens the same way you would with a camera lens. If the mark still persists, try cleaning the lens with
a damp lint free cloth in a gentle circular motion, as you would with a camera lens. Stubborn marks will require some pure Ethanol wiped on with a lens tissue and then whiped off with a new lens tissue. The trick is not to leave a smear. Never take the lens apart to clean it!
Cleaning the mirror in a reflector telescope such as my Dobsonian is a relatively major operation that should be undertaken rarely(remember you can have about a 20cent piece worth of combinined dust on the mirror before you need to clean it).Before removing mirror base scribe a fine alignment line (I always scibe 3 and number them accordingly but this is just for more accurate alignment on reassembly).
First remove the mirror in one piece from the rear of the telescopes main tube, there is usually 6 self tapping screws around the outside that need to be removed and omly these screws. If some resistance is felt just slowly tap around the base of the mirror with a drift and a soft faced hammer, this is not a slogging match, take your time or you will end up doing damage to your pride and joy and end up doing a whole lot more than you initially bargined on. Remember the mirror has an aluminium coating which means you can easily scatch it, be careful!!
Have a large bowl or clean sink of warm soapy water ready with another bowl/sink with just plain warm to hot water to wash and rinse mirror. Move mirror in a gentle manner in the soapy water then transfer to clean water to rinse, remove any drops by touching not rubbing with a small lint free cloth or lens tissue. Reinstall the mirror is reverse of extraction. Good luck and pleasant viewing, Nic.