Most Weather Service reports of barometric pressure are adjusted from the observing station elevation above sea level to a value at sea level.  This is done in order to put the data from a collection of stations on the same basis for analysis.  If it were not done, it would be necessary to consider the elevation of each station in the analysis.

Three data values are needed to convert local pressure to sea level pressure:

1. The elevation of the barometer above mean sea level.  Being from the US, I use feet.  This does not change from reading to reading unless the barometer is moved.
2. The outdoor temperature at the weather station site.  Being from the US, I work in degrees F.
3. The outdoor relative humidity at the weather station in percent.

The pressure adjustment is made by considering an imaginary column of air extending from the barometer to mean sea level.  The weight of the air in this imaginary column is used to calculate the sea level pressure from the local pressure using the hydrostatic equation.  In order to factor in the effect of humidity, the virtual temperature is used to calculate the weight of air.  The virtual temperature is the temperature of dry air that would have the same density as the humid air.  

The equations for calculation of sea level pressure from local pressure, along with numerical examples of the calculation, are shown in the following PDF document (3 pages):