Question
whatneeds to be done to the hydranger plant to change the color from white to blue someone said this can be done but how

Answer
There are different types of hydrangeas.

The white types are typically arborescens or quercifolia(such as "Annabelle" or "Snow Queen"). These hydrangeas bloom naturally white and this will not change.

Other hydrangeas such as Hydrangea Macrophylla ("Mopheads") will most often change color based on the soil pH of the soil. These hydrangeas will typically bloom pink, but if the soil contain large amounts of aluminum the plants will flower blue.

In soil with very low soil pH the heavy metals such as aluminium is easier obsorbed by the plants and thus there is a direct correlation between low soil pH (acidic soils) and soil pH. If your soil is very low in aluminum it is of course possible that even with low soil pH there is not enough to feed the plant, but since this nutrient is taken up in extremely small amounts this is typically not a concern.

You can create an environment for hydrangea plants to bloom light blue if you mix in plenty of leaf mold or peatmoss sphagnum in the planting hole to provide a low pH. You can also change the soil pH by topdressing with aluminum sulfate or gardeners sulfur.

(Aluminium sulfate is "aluminum" + sulphur). Sulphur will lower the soil pH. The advantage to using aluminium sulfate is that you directly add the aluminum besides lowering the soil pH so it can be taken up easily. Sulfur alone should normally be enough if the soil has adequate amounts of aluminum.

However, as said, white flowering hydrangeas usually do not bloom blue irrespective of soil pH.