Upstairs and downstairs are not used with to/in/at etc. They are mainly adverbs, but they are also used as adjectives and nouns.
- He went upstairs to answer the phone. (NOT to upstairs)
- The people who live upstairs are very noisy.
- My room is upstairs (NOT in upstairs)
- The dining room and kitchen are upstairs. (NOT in upstairs)
- The upstair(s) rooms. (often without the final ‘s’)
- She’s cleaning the upstairs.
- The upstairs of this house is for sale.
The above rules are equally applicable to ‘downstairs’ also.