Update value of variable

Hallo all..
I'm running in windows 7 and cygwin instaled so i can use linux command on that.

I want to update the value of some variable that i stored in a file. How do I update it.

this is my file look like..

variable.txt
partisi1=25
partisi2=25
partisi3=25
partisi4=25

how can i change the value of the partisi1 variable by shell script?
thank before..

Take a look at sed command.