By changing the two most fundamental factors Pressure and temperature which are for any state of the body wether it is liquid , gas or solid.
Hence By keeping the pressure constant  and by increasing the temperature of that body they will be converted to liquid state and they remain in liquid state at a particular temperature.
As the temperature is the measure of the kinetic energy of molecules of any body.
So if we will increase the temperature by heating it the molecules gain energy and tend to away from each other.Like this the kinetic energy increases and they are converted  to liquid state.I hope u got it!

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