hi,
The normal way to do it is
Go to settings - security - users - search for that user name & open the system user record.
You will find the “Change Business Unit” in Ribbon/command bar. Use it to change his BU.
If you have huge number of users you can use a workflow:
Create a checkbox field or any other field in user form, then add any value in that field or check the check box for the users you want to change the BU.
Create a on demand workflow which will change the BU of the users for which the field is checked or meet a certain value in any specific field.