add a floating point variable called VoltagePrev or whatever
measure the voltage.
if measured voltage different than previous voltage :
go to position where the voltage is printed
print previous voltage with the background color to erase the previous reading
go to position where the voltage is printed
print new voltage with the text color
set the previous voltage value to the new voltage value
end if
When the voltage is measured again, the previous voltage variable holds the voltage used last time the screen was updated, so if the measured voltage is the same as last displayed voltage, you're not printing anything.
By the way... see datasheet of that ADS1115 :
http://www.ti.com/lit/ds/symlink/ads1114.pdfGo to page 17 : 9.3.3 Full-Scale Range (FSR) and LSB Size
If you don't need that full range, you can set the maximum to 4.096v which gives you 125uV per bit so that would save you a floating point division ... 8 units give you 1mV so you could simply shift right by 3 bits
ex ... if you read 1200 ... 1200 >> 3 = 150mV