When investing, it's better to become discrete when it comes to the ups and downs of the stock price.
ご質問ありがとうございます。
この場合「[株価](https://eikaiwa.dmm.com/uknow/questions/66015/)の上下に一喜一憂せず粛々と投資するといい」と言いたいなら、英語で「When investing, it's better to become discrete when it comes to the ups and downs of the stock price.」と言えます。
ご参考になれば幸いです。
"You shouldn't be too concerned with the fluctuation of stock prices when investing."
"You shouldn't be too concerned with the fluctuation of stock prices when investing."
"you shouldn't ~" 「〜しないほうがいい」
"to be too concerned with ~" 「〜気にしすぎる」・「に一喜一憂する」
"you shouldn't be too concerned with ~" 「〜に一喜一憂せず」
"the fluctuation of stock prices" 「株価の上下に」
"when investing" 「投資する」
[これも](https://eikaiwa.dmm.com/uknow/questions/103263/)役に立つかもしれませんので是非クリックしてください。