。
She remade her old dress into a skirt.
Literal
She [topic-は] old dress [object-を] skirt [into-に] altered.
直す has a wide semantic range — 'fix, correct, mend, rebuild, alter' — and in clothing contexts it specifically means to alter or remake a garment. The に here marks the resultant form: 'altered the dress INTO a skirt,' the same construction as 部屋をきれいにする ('make the room clean'). Repurposing old clothes by hand fits a strain of Japanese household culture sometimes labeled もったいない — the principle of avoiding waste by giving worn or outgrown items new life. The same instinct underlies sashiko mending and the long tradition of remaking adult kimono into smaller children's garments.