Patience is key!
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 in including a high enough miner fee 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 can not cancel an unconfirmed transaction, neither can ShapeShift. We can cancel the exchange at your request- but can only return your input coin once confirmed.
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 can not 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 be returned to your wallet.
We suggest you use our tool to determine the most advantageous fee for your transaction: https://shapeshift.io/#/btcfee