Your print dimensions are linked, so when you change one dimension the other adjusts automatically to keep your artwork's proportions intact.
Why the print dimensions are linked
creativehub reads your artwork's aspect ratio — the proportion of width to height, such as 3:2, 4:3 or 1:1 — from the original image file you uploaded, and locks it in. If you could set both dimensions independently, your artwork would end up stretched or cropped, so creativehub adjusts one dimension to match the other.
If you need a different aspect ratio
To change the proportions, crop or resize your original image file before uploading it, then upload it to creativehub as a new artwork.