How do you step up amperage yet reduce voltage in a dc circuit?

Use an inverter. In this case they are often called DC-DC converters. It is the DC equivalent of a plugpack.

It converts the DC to AC so it can pass through a transformer, then converts it back to DC. The operating frequency is generally tens of kHz or higher so that a small transformer can be used. There are many kinds, starting with say 5 or 12 volts, and providing other voltages.

The prices vary depending on the purpose. High reliabilty models (for telecommunications, military, etc.) and certified models for fire and insurance tend to be expensive. Devices with low electrical noise is also likely to be expensive because of the extra design effort and filtering and testing.

If it is to convert 24 volts to 12 V (as often required on trucks, boats and such, there is a specific market. More.

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