Go to Settings > Passwords & Accounts, then tap your Exchange account.
Turn Contacts off, then tap Delete. This removes the Exchange contacts on your device that you'll download again in the next step.
Turn Contacts on.
Contacts from your Exchange account automatically redownload to your device with the correct birthday.
Information about products not manufactured by Apple, or independent websites not controlled or tested by Apple, is provided without recommendation or endorsement. Apple assumes no responsibility with regard to the selection, performance, or use of third-party websites or products. Apple makes no representations regarding third-party website accuracy or reliability. Contact the vendor for additional information.