All you have to do is buy a reserved instance of the same type as the current running instance.
How it works, AWS compares the current list of your reserved instances with working ones and will pay for the activity at the price of the reserved instance for several running instances, equal to the number of reserved instances purchased.
For example: you have 4 running instances of c1.xlarge. You buy 2 c1.xlarge reserved copies. From now on, AWS will pay for activities in this way: 2xc1.xlarge at a reserved price, 2xc1.x more at a price on request.
Hope this is clear enough let me know in the comments.
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