Unless the vacuum is very old, has several broken parts or has parts that consumers can only replace at a service center, it's generally cheaper to repair the vacuum. Dyson sells many parts that consumers can replace themselves, such as belts and hoses.
Dyson makes a variety of vacuum cleaners, including uprights, canisters and cordless models. Before thinking about a repair, check that the brush bars and hoses aren't blocked or need hair and lint removal. Also make sure that the filters are clean and dry. Read the instruction manual for model-specific instructions.
If the vacuum is still under warranty, contact Dyson to arrange a repair. Note down the vacuum serial number before buying any parts. Many spares are available to buy on the Dyson website, but parts for older models may not be available after Dyson discontinues them. Dyson offers telephone support and has some service centers in the United States.