Price converted wrong in cart

I have multiple currencies enabled, the converted price in cart and checkout page is just not right, seems it converted twice, and I found the following link as resolved.

https://www.ads-software.com/support/topic/currency-conversion-is-wrong-in-cart-page/
However, I am new to this plugin, can anyone tell me where is this function.php file located, please give me the exact folder path and file full name. thanks.

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