Here[0] is a much larger (2000x1333) image of the letter from an article[1] in The Japan Times for those who love period details like fonts, telegraphic addresses, penmanship, etc.

And the tl;dr:

"Stilles bescheidenes Leben gibt mehr Glück als erfolgreiches Streben, verbunden mit beständiger Unruhe."

translated as:

"A quiet and modest life brings more joy than a pursuit of success bound with constant unrest."

[0] https://www.japantimes.co.jp/wp-content/uploads/2017/10/n-ei...

[1] https://www.japantimes.co.jp/news/2017/10/22/national/einste...