Bitcoin transactions are processed by the miners. Block sizes are limited, so a limited number of transactions can be confirmed in each block.
The easiest way to speed up a transaction is to be proactive. Include a high miner fee so that your transaction is worthwhile to the miner.
Alternatives include rebroadcasting the transaction or using a transaction accelerator (beware of scams). Here are some resources to review:
You cannot cancel an unconfirmed transaction, and neither can ShapeShift.
If your transaction confirms within 48 hours, your exchange will complete normally.
At the 48-hour mark, the exchange will cancel in ShapeShift’s system. Once the funds confirm, we will be able to return your funds to you. We cannot process any returns until the funds confirm in our wallet.
If the funds do not confirm in 72 hours, the mempool may drop the transaction and the coins should return to your wallet.