alterar

To alter or change something, often in a way that disrupts or disturbs — alterar el orden, alterar los planes — and, for people, to upset or agitate: la noticia la alteró mucho. Rarely used for small, neutral tweaks the way English 'alter' sometimes is; it leans toward disruption.

• El escándalo alteró por completo la campaña electoral. — The scandal completely upended the election campaign.
• No te alteres, todo tiene solución. — Don't get upset, everything has a solution.
• La lluvia alteró los planes del festival. — The rain disrupted the festival's plans.

Using alterar for a small, neutral edit (like tailoring a piece of clothing) — that narrower sense is covered by arreglar or ajustar in Spanish; alterar almost always implies disturbance or upheaval, not a minor tweak.