Hi Frank,
Which i.mx6 processor are you using? Firstly, you need to confirm if it support 1.2Ghz work.
About how to change you can refer to the follow thread:
How can I change CPU frequency on i.MX6Q ?
Change CPU frequency
https://community.nxp.com/message/662973
Hope this can help you. Any questions you can conact me.
Have a great day,
Rita
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