1. Under “Administer,” select Security Controls –> Login Access Policies.

2. Select Administrators Can Log in as Any User.

Note:
If Administrators Can Log in as Any User is not found, contact Salesforce.com support.

Check the below link for more information

https://help.salesforce.com/articleView?id=Enabling-Organization-Admins-Can-Login-as-Any-User&type=1