The best thing to do is to stay put and do not transact bitcoins during this period.

The following post outlines the measures ShapeShift is taking once the BIP 91 Activation takes effect, around midnight Saturday (July 22, 2017)/ Sunday (July 23, 2017) morning EDT at earliest. Due to an abundance of caution regarding the activation of BIP 91 on Bitcoin, ShapeShift will temporarily remove all trading capabilities with Bitcoin on our exchange – and within our API offerings – in order to ensure the orphaning of blocks causes no major consensus issues. No deposits or withdrawals will be available at the time that this activation occurs until we feel that a clear outcome has been determined. HOWEVER, ShapeShift will continue to support all non-Bitcoin transactions. Read more to learn all details.

Source: Notes on ShapeShift Preparing for BIP 91 (Segwit2x) Activation | ShapeShift