。
On June 10 this year, she'll have been married for six years.
Literal
She [topic-は] this-year 's June 10 [at-で], married-and 6 years becomes.
The pattern [te-form verb] + [duration] + になる is how Japanese expresses elapsed time: literally 'having married, becomes 6 years.' English has to flip it into a perfect tense ('will have been married'), but Japanese keeps it as a simple becoming. The particle で here is the で of a punctual time-point — 'as of June 10' — different from the locative or instrumental で. Numerals like ６月１０日 in Japanese text are typically written in full-width digits when alongside Japanese script.