1. Enable Mailbox Replication Proxy Service on Client Access Servers
Before conducting hybrid migration, it is suggested to create migration end points. A migration end point has the login credentials and connection settings of the source Exchange Server. Then follow the steps to enable the Mailbox Replication Proxy Service (MRSProxy service):

In Exchange Admin Center, go to Servers > Virtual Directories
Choose the Client Access Server, select the EWS virtual directory, and click Edit
Enable MRSProxy (by checking the checkbox) and click save

2.Migrate Exchange 2013 Mailboxes to Exchange Online
In the Office 365 tab of the Exchange Admin Center, you have the remote move migration wizard that helps you move mailboxes from on-premises Exchange to Exchange Online (and vice versa). Follow the steps below to migrate on-premises Exchange mailboxes to Office 365 (Exchange Online).

To know more about Migrate Exchange 2013 to Exchange Online, visit O365CloudExperts.