Answer:
Reducing or increasing the amount of H+ ions / hydronium (H3O+) ions
Step-by-step explanation:
To reduce the pH (reducing the strength of the acid) can be done by adding a base (including a conjugate base such as bicarbonate ion) which will absorb the H+ ions either through adsorption or reaction.
Adding more H+ decreases the pH of the acid making it stronger. This can be done by adding HCL that will dissociate and increase the H+ ions.