find and replace a value in a line of text

I have a text file which contain only 1 line of text

1314:Battery:1.90:50:30

I'm trying to figure out if is there anyway to do a grep and sed to change the value 50 and 30 to the following

1314:Battery:1.90:45:35

which I would reduce 5 from 50 and increase 5 from 30.

Please guide me if there is a better way to solve this problem ? I just started picking up this scripting. Thanks =)