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let go
1
: to dismiss from employment <the firm let him go at the end of the month>
2
: to abandon self-restraint : let fly <spoke in clipped sentences, as if fearful of letting go - David Kline> <there just to party, just to let go - Philippe Vergne>
3
: to relax or release one's hold -used with of <let go of stress - Kathy McCoy> <let go of my arm>
4
: to fail to take care of : neglect <let himself go and got real fat - Bill Parcells>
[M-W's Col. Dic.]

How would you read that definition like 'to relax one's hold/to release one's hold' or some other way?

"let go of stress" - (Please you) let go of stress. "you" is not holding on his stress, then how can he let it go?

Please help me.
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Sorry, I don't understand your first question.

As for the second: yes, he has stress, so he can release it (= let it go). 'Please you' does not work, though. Please let go of your stress. Or more naturally: Please relax and let go of all that stress you've built up today.
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Mister Micawberyes, he has stress, so he can release it (= let it go).
"let go of stress" - (Please) let go of your stress.

But "you" is not holding on his stress, then how can he let it go/release? The word "hold" suggests as though the grasp on something is intentional but in the case it would be wrong because no one likes to be stressed. Please guide me on this.
You have asked the same question twice. I have already answered it in my post #1199714. Once again, you have not used any of the dictionaries I have suggested to examine the meanings of 'hold', but have decided to further usurp my time. Consider this sentence:

The canteen holds fresh water.