Your first step should be to reset your scale so the scale reads zero.
Your scale may feature a special code to reset it to factory conditions. For example, some models require stepping on a specific corner two or three times to clear all digital codes and return the scale to zero. You can check this information in your scale's user manual.
Once the scale is successfully set to zero, test the scale's accuracy with an object of a known weight. Something like a 10-pound free weight is ideal and should let you know immediately whether the scale is measuring and displaying weight accurately. If so, the scale should also reflect heavier weights accurately as well.