Becquerel’s experiment consisted of four steps. 1. get a sample of a uranium salt. 2. expose the salt to sunlight. 3. expose photographic plates to the salt. 4. develop the plates. a change in which step led to becquerel’s discovery of radioactivity?

A change in STEP 2 led to Becquerel's discovery of radioactivity.
Becquerel's intention was to carry out his experiment on a sunny day in order to get a lot of sunshine. But on the day he intended to carry out the experiment the weather was cloudy, the weather was not sunny at all, thus he was not able to carry out the experiment. He wrapped the uranium salt in a black cloth along with the photographic plate and a copper cross and stored them in a dark drawer. The next time he brought these material out he discovered that  a distinct image of the cross appear on the photographic plate without been exposed to the sun. This led to the discovery of radioactivity. 

A change in STEP 2 led to Becquerel's discovery of radioactivity. When the experiment was made, instead of a sunny say, it was on a cloudy day. So from the weather condition, he wrapped his subject and then do his process and found out something new. This was the start of the discovery of radioactivity.