Simply replacing the 15-A fuse with a 25-A fuse allows more current to flow. The wires in the circuit are not designed for this kind of load, and they will get hotter than normal, possibly resulting in a fire or other damage.
Work Step by Step
There might be a “short circuit” or some fault causing the current to exceed 15 A. That’s why the 15-A fuse blows. The blown fuse is the canary in the coalmine, and you must track down what is wrong with the circuit.