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I don't understand about meaning of word of the phrase: "make up someone's mind".

For example: he makes up his mind about something.

Thanks you for helping.

"to make up someone's mind" has two main senses:

1) to decide something about which they are or were unsure about.
2) to make a way of thinking firm and unlikely to change.

I couldn't make up my mind whether I wanted tacos or burritos.

The writer has a choice to make but is having trouble making the choice.

She hadn't made up her mind whether to accept his marriage proposal or
not, until she thought she had lost him.

Another example of a choice, this time with the reason for the decision included.

It's useless to try to reason with him once he has made up his mind.

This is the second sense of the word, where a person's way of thinking becomes rigid and not easily changeable.

She had made up her mind that her boyfriend was cheating on her.
Nothing he said could convince her that he was innocent.

Another example where the phrase means that a person won't change their thinking.

It means "to decide":

If you make up your mind or make your mind up, you decide which of a number of possible things you will have or do.

https://www.collinsdictionary.com/dictionary/english/to-make-up-your-mind