In "Tangent Twins" building, where you choose the arena, there will be a "Reroll Battles" option at the top. You get rerolls as you perform battles, and each reroll would require to spend at least some amount of time fighting bytes. Amount of rerolls is capped at 99.
Otherwise, battles and weapons get rerolled after 8 battles (challenges excluded), but that also ultimately resets a day in Glitch City.