I was going to say your cables probably need to be replaced, but never beat me to it!
It probably just needs a good tune-up.
www.parktool.com and go to the repair guide for DIY.
If it shifts fine in 'lets say' the bottom half of the gear range, but not the top, then your derailleur hanger is probably bent. You can usually straighten them up if not too bad of shape, but I always buy two at a time. That way I always have (and carry) a spare with me.
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"Life is not a journey to the grave with the intention of arriving safely in a pretty and well preserved body, but rather to skid in broadside, thoroughly used up, totally worn out, and loudly proclaiming, "WOW, What a Ride!" - unknown
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