Start by lifting the grille and reading the fan you have. Note the housing size, the mounting bracket style, and whether the motor is a separate unit or part of a sealed module. Write the model and serial from the label before you remove anything. A replacement that matches the opening is easier to plan than one you have to adapt on the ceiling.
Check the duct path first
Follow the duct from the fan housing to the outside. Look in the attic or crawl space where you can reach it. Note the duct diameter, whether it is rigid or flexible, how many bends it takes, and how long the run is. A long or flattened duct moves less air than the label suggests, and a new fan will not fix that. If the duct ends in the attic or in a soffit instead of a roof or wall cap, the moisture problem is a separate job, and it belongs on your list before the swap.
Check the switch circuit
Find the wall switch and note whether it controls only the fan, or the fan and a light. Note whether the fan has a separate receptacle or is hardwired. A swap that keeps the same switch and the same wire gauge is a different job from one that adds a timer, a humidity control, or a second light. Any change that adds load, moves a box, or touches a shared circuit is the point where a licensed electrician is the usual route. If the circuit looks old, cloth-jacketed, or undersized for the new unit, the electrical panel page on this site explains how panel age is planned, and that can help you decide whether the panel itself needs a look.
Check the roof or wall cap
Go outside and look at the vent cap. Check that the damper flap opens freely and closes under its own weight. Look for staining on the ceiling around the fan opening, which can mean the duct has been dripping condensation for a while. A roof cap often needs ladder work, and that is a fall risk. If the cap has to be rebuilt or the roof opened, that is a trade job in most cases.
Decide who does what
A plain like-for-like swap with a matched housing, a working switch, a clear duct, and an easy outside cap is a reasonable owner task for someone comfortable with basic household wiring and ladder work. Turn the breaker off, confirm the power is dead with a tester, and keep the old and new wire connections labeled. Anything that adds a circuit, changes the duct route, or needs a permit should go to a licensed hand. Your local building office can tell you whether a permit applies to your fan work and what inspection it needs.
What to ask a hired tradesperson
Ask for the scope in writing. It should name the fan model, the duct work included, the switch and wiring work included, whether the old unit is removed and hauled away, and who patches and paints the ceiling opening. Ask whether a permit is pulled and who pulls it. Ask how the work is inspected, if at all, and what the paperwork says about the new unit and the labor. Read any warranty papers for the fan and for the installation separately, because they often start on different days and cover different parts. If the paper does not name the duct, the cap, or the switch, ask where those parts fall.
Before you book
Have the duct path, the switch details, the cap condition, and the fan label in hand when you call anyone. That list lets you compare two written scopes on equal terms. The Walk-through section of this site covers the seasonal checks that often find the same duct and cap problems early, and it is a useful next step if the fan is part of a larger house check.