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How to uninstall MS Exchange Server 2010
In addition to PeterRising recommendation see the following articles providing more details:
Office 365 and Dirsync: Why should you have at least one Exchange Server on-premises
https://blogs.msdn.microsoft.com/vilath/2015/05/25/office-365-and-dirsync-why-should-you-have-at-least-one-exchange-server-on-premises/
How and when to decommission your on-premises Exchange servers in a hybrid deployment
https://technet.microsoft.com/en-us/library/dn931280(v=exchg.150).aspx
Decommissioning your Exchange 2010 servers in a Hybrid Deployment
https://blogs.technet.microsoft.com/exchange/2012/12/05/decommissioning-your-exchange-2010-servers-in-a-hybrid-deployment/
VickVega Decommissioning your Exchange 2010 servers in a Hybrid Deployment
Above link not working and looking for the steps?
aussupport
Here is the updated link. Since Microsoft changed platform, those articles can be found by plugging the title of the article in search. It should get you to the updated location.