A 61-key keyboard has a range of 5 octaves plus 1 note, which is only 2 octaves (and a bit) short of a full 88-key keyboard, and enough for most pieces of music.
If you just need to shift the range, say add an extra octave in the bass (at the expense of the treble), then the easiest thing to do will be to use the transpose function on the keyboard. Most keyboards have a transpose function that shifts the sounds of the keys up or down by up to an octave (or possibly two).
However, if your sheet music has very high notes and very low notes, you'll need to transpose one of the parts (one of the voices/hands) up or down an octave so that it fits into the 5 octave range. (It's possible that you only need to transpose some phrases of the music.)
If that doesn't work you could try transposing the sheet music to a different key, and then try some of the techniques above. But it's hard to be specific without seeing the actual music you are working with.
Thank you for linking to the score you are working with — it certainly makes it easier to see the specific problem.
That particular arrangement has lots of areas that would require special attention to make it playable on a 61-key keyboard. I would suggest looking for a different arrangement as a starting point.
This arrangement by Hector Duran (also on MuseScore) is a bit simpler. (It's not a perfect arrangement, the last line (mm. 56-60) seem to have an engraving mistake, but that's rather trivial to fix.)
A quick glance tells me that in Hector's arrangement there is only one note (an A1 in m. 13) that would be below the standard 61-key keyboard range. However this is a very significant note in the piece. I'd suggest replacing this note with an octave "chord" an octave above what is written (A2 & A3 simultaneously), and to play it with an accent.
Alternately, if you can use the octave shift/transpose function, there'd be only one note (an E6 in m. 17-18) above the range. I'd suggest you play this note as well as the preceding note an octave below (as an E5, as an E4 respectively).
If you really prefer the Torby Brand arrangement, it would be a good idea to compare these two and the original (and other) arrangements closely — and see how the various arrangers have handled the various passages.