255 would be '2' '5' '5' '0'
The zero marks the end of the string. If you are parsing a longer string then after 255 there would be a ' ' (space) that you would look for to maybe replace with 0 but if you want to parse the rest then save that location to a pointer so you can go back and do that.
If you don't know what pointers are then you need to search on C pointers and do some learning. Nothing I could post quickly here will just give it to you right away, learning this properly means doing some homework with sketches.
You can turn '2' into 2 through val = '2' - '0' but check that you have '0' to '9' -- always expect that an error will happen, serial data has no guarantees.
Normally I would pull each character in, check (if error then report it and either stop the program or ask for input again), multiply any old value by 10 (initialized to 0, 0 * 10 = 0) and add the new value until I hit the terminating zero.
But there's at least 3 other ways including using atoi() which can return error as well.
Even parsing, you can use strtok() which is very nice but actually more complicated than processing a string as an array which is funny because so many people use it "because it's easier". Yeah sure, -after- you learn the ins and outs there's less typing so that makes it easier. Or don't learn the ins and outs and spend even more time puzzling out bugs.
You have a few things to learn no matter what. Like how C strings work, what ASCII code is, and pointers. Then have a look through C string commands which once you the first stuff should make easy sense.
After that, plan your code.
Here's another clue; if you don't understand everything about example code then find out before you go modifying it or using pieces of it in something else.