。
She put fresh sheets on the bed.
Literal
She [topic-は] bed [on-に] clean sheets [object-を] spread.
敷く ('to spread, lay out, put down') is a tactile verb used for spreading flat materials over a surface — sheets, futons, mats, even tablecloths. The に marks the bed as the surface receiving the sheet. きれいな here means 'clean, neat' rather than 'pretty' — きれい spans both senses, with context disambiguating. Two katakana loanwords appear (ベッド and シーツ); both came in via early Western contact and are now thoroughly naturalized.