In general terms, revaluation of a currency is a calculated adjustment to a country's official exchange rate relative to a chosen baseline.
The baseline could in principle be anything from wage rates to the price of gold to a foreign currency.
In a fixed exchange rate regime, only a decision by a country's government (specifically, its central bank) can alter the official value of the currency.
However, the central bank may experience political pressure from two sources to increase the value of the currency: Domestic consumers will complain that they find it expensive to acquire foreign currency with which to buy importable goods; and foreign governments, on behalf of foreign exporters, may urge such a revaluation to improve their countries' sale of exports.
A revaluation of the local currency to a higher value vis-a-vis other currencies will make it less expensive for local consumers to acquire the foreign funds with which to import foreign goods, so they will do more importing.