We cannot translate it by a word. Depending on the sentence, we need to change it. Let’s see with example sentences.
doubutsunomori wa, hinichi wo kaenaide ichinichi zutsu yarimashou!
You should play Animal Crossing day by day without changing the dates.
ichinichi zutsu. (shikkarito ikiteikou!)
*inside the parenthesis is an implication.
One day at a time.
Un jour à la fois.
This phrase is not the only translation of ‘One day at a time’, however when I felt stressed about immigrating to Canada (to a French speaking area. I could not speak any French at all at that time.) and I cried, my mother-in-law gave me the phrase ‘Un jour à la fois’. In that situation, I understood that she said the sentence as I wrote above.
This is one of the 四字熟語 (よじじゅくご / yoji jukugo = Four character idiomatic compounds).
ichinichi ippo, seichou shiteiku.
Growing (improving) day by day.
haisha-san no eigo ga, hinihini yokunatte imasune!
Haisha-san(my beaver plush doll), your English is getting better day by day.
oyashirazu no seide, hanarabi ga hinihini warukunaru.
Because of the wisdom teeth, my teeth alignments get worse day by day.