YOU SAID:

I am walking up the stairs.

INTO JAPANESE

私は階段を上っています。

BACK INTO ENGLISH

I'm walking up the stairs

INTO JAPANESE

階段を上っています

BACK INTO ENGLISH

going up the stairs

INTO JAPANESE

階段を上る

BACK INTO ENGLISH

climb the stairs

INTO JAPANESE

階段を上る

BACK INTO ENGLISH

climb the stairs