How does the voltage increase or decrease with current in a transformer?

Hint :A transformer is a passive electrical device that transfers electrical energy from one electrical circuit to another, or multiple circuits. A varying current in any one coil of the transformer produces a varying magnetic flux in the transformer's core, which induces a varying electromotive force across any other coils wound around the same core. Electrical energy can be transferred between separate coils without a metallic (conductive) connection between the two circuits.

Complete Step By Step Answer:
As voltage is increased, the current decreases accordingly.
Similarity, as voltage is decreased, the current accordingly increases.
This happens to conserve electrical power and hence energy.
The electric power is given by,
P=VI
Now, there is no power source which can increase the power and hence power must be conserved (not considering the dissipative losses for now).
Thus, as V goes down I go up and vice versa.

Note :
A transformer whose primary coil has more turns than its secondary coil is called a step-down transformer because it reduces voltage — that is, the voltage at the secondary coil is less than the voltage at the primary coil. Similarly, a transformer that has more turns in the secondary than in the primary is called a step-up transformer because it increases voltage.