The magnification of a telescope is worked out by dividing the focal length of the telescope by the focal length of the eyepiece in use.
So, for example, a telescope with a focal length of 750mm used with an eyepiece of 15mm focal length will have a magnification of 50x.
You cannot alter the focal length of a telescope, but you can alter the magnification by changing the eyepiece to one with a longer or shorter focal length.
With the same telescope, an eyepiece with a focal length of 10mm would yield a magnification of 75x, while an eyepiece with a focal length of 25mm would give 30x.
For more info, read out guide to telescope stats.
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