Replace variable resistor

I think you are looking for a digital potentiometer.
u can do this with IC, X9015US
when u send a positive pulse at the input, it increase the resistance
whereas when u send a negative pulse, it decrease the resistance.
The total resistance can be adjusted. Sorry it is hard to explain in here. Please find the detail in the datasheet below, or simply type X9015 in yahoo search.

http://www.datasheetcatalog.com/datasheets_pdf/X/9/0/1/X9015US.shtml

Variable resistors consist of a resistance track with connections at both ends and a wiper which moves along the track as you turn the spindle. The track may be made from carbon, cermet (ceramic and metal mixture) or a coil of wire (for low resistances). The track is usually rotary but straight track versions, usually called sliders, are also available.
Variable resistors may be used as a rheostat with two connections (the wiper and just one end of the track) or as a potentiometer with all three connections in use. Miniature versions called presets are made for setting up circuits which will not require further adjustment.

Variable resistors are often called potentiometers in books and catalogues. They are specified by their maximum resistance, linear or logarithmic track, and their physical size. The standard spindle diameter is 6mm.




Standard Variable Resistor
Photograph © Rapid Electronics