Question

some one wrote this letter in the nineteeth century ( voice change)

Answer 1

This letter was written in the nineteenth century (by someone).
I've added 'someone' in brackets bcoz it isn't needed here, but if u write it in the answer, it will still be correct.

Answer 2

passive voice::::
The letter was written in the nineteenth century......

" by someone " needn't mention here... but if you add it, the sentence will be correct....