Visito la sucursal nueva por la mañana.

In Spanish, when you talk about an unspecified time during a part of the day (“in the morning,” “in the afternoon,” “at night”), you normally use por + part of day: por la mañana, por la tarde, por la noche.

• En la mañana is less common and can vary by region (more typical in some Latin American countries), but por la mañana is safest.
• Just mañana means “tomorrow,” not “in the morning.”

Spanish uses the simple present (pretérito imperfecto de indicativo) for near-future or scheduled events, especially in timetables or personal plans:

• Mañana visito al médico. (“Tomorrow I visit the doctor.”)
  It’s perfectly normal to say Visito la sucursal nueva por la mañana to mean “I’ll visit the new branch in the morning,” as long as the context makes the timing clear.

Visitar is a transitive verb that takes a direct object without a preposition when that object is a thing or place:

• Visito la sucursal
  You only use the “personal a” with visitar when the direct object is a specific person or pet:
• Visito a mi abuela
  Since sucursal is not a person, no a is needed.

Both are grammatically correct:

• La nueva sucursal (adjective before noun) treats “new” as an inherent quality—“the (brand-)new branch.”
• La sucursal nueva (adjective after noun) has a slightly more “identifying” or restrictive feel—“the branch that is new (as opposed to the others).”
  In everyday speech, most people wouldn’t stress a huge difference; it’s largely a nuance.

Using la (definite article) signals that you and your listener both know which branch you’re talking about:

• Visito la sucursal nueva = “I visit the new branch (we both know which one).”
  If you said una sucursal nueva, it would mean “I visit a new branch” (we don’t know exactly which one—just any branch that happens to be new).