The key question is "what do you mean by improve"?

I'd start by asking "what are you doing that you will still have to do in

part 2, and what won't you do?"  So looking at part 2, you will want to

convert the lines to integers, and

  input := Array streamContents: [:lines |

    'input.txt' asFileReference readStreamDo: [:in |

      [in atEnd] whileFalse: [lines nextPut: in nextLine asInteger]]].

gives you a chunk of code you can use in both parts.  So you might

want to have

Day1

  changesFrom: aFileName

    ^Array streamContents: [:changes |

      aFileName asFileReference readStreamDo: [:in |

        [in atEnd] whileFalse: [changes nextPut: in nextLine asInteger]]]

  partOne: aFileName

    ^(self changesFrom: aFileName) sum

  partTwo: aFileName

    ...

The file name should not be wired in because you want some test files.