She is changing clothes upstairs.

她正在楼上换衣服。

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Moira walked upstairs to change her clothes.

莫伊拉上楼去换衣服了。

《柯林斯英汉双解大词典》

He went upstairs and changed into clean clothes.

他上楼换了干净的衣裳。

《柯林斯英汉双解大词典》

Now, once again, the property looks as if Alfred Lunt and Lynn Fontanne were just upstairs changing into evening clothes.

NPR: The 'Fabulous Lunts,' Alive at Ten Chimneys

In the days when Costigan was more lively she had enjoyed these moments of deception, the exact change put down on the table, what she had saved still secreted in her clothes until she could get upstairs to the Gold Flake tin.

NEWYORKER: The Woman of the House

If you're upstairs, you don't have to change your clothes five times a day.

WSJ: The Weekend Interview with Julian Fellowes: The Anti-Snobbery of 'Downton Abbey'