Upstairs and downstairs are not used with to/in/at etc. They are mainly adverbs, but they are also used as adjectives and nouns.

As adverb:

• He went upstairs to answer the phone. (NOT to upstairs)
• The people who live upstairs are very noisy.

As adjective:

• 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’)

As noun:

• She’s cleaning the upstairs.
• The upstairs of this house is for sale.

The above rules are equally applicable to ‘downstairs’ also.