You spell resume in a cover letter the way it’s spelt in your target job advert: either with accents (résumé) or without accents (resume).
If the job advert doesn’t have the word ‘resume’, use the version you prefer or use ‘CV’ instead when you write your cover letter. Even if you’re applying to a foreign company that requires a resume instead of a CV, the recruiting manager will know what a CV is.
