Hi @Perumal ,
Please note that for a person column, it's just displaying people names but backend it includes an object of user info. On the other hand, populate a single selection person column requires the Claims info of a user.
To your question, you need to get all items that Manager is "Mike" first, then update each item of the result, populate the Manager column with another person's Claims in the specific format:
i:0#.f|membership|EmailAddress
So, if you only know the name of new user, you will have to search for use by the name and get their Email address, use in Update item action:

Details:



Filter query:
substringof('OldName',Manager/Title)
New Claims:
i:0#.f|membership|@{items('Apply_to_each_2')?['Mail']}
Best regards,