The easiest approach in MATLAB is to read the entire file, change the last line, and write out the result to the original file name.
This has a bit of risk in that if something goes wrong during the writing, then you could end up with a file that is incomplete.
You might think that surely there must be a way to only change the last line. Unfortunately, the way that text files are structured in any operating system you are likely to ever use, the operating system is not keeping track of where the line boundaries are, and the best you can do is use line reading and file information operations to start at the beginning of the file and progress a line at a time figuring out where the lines are, then tell the file system to move you back to the beginning of the last line (based on the information about position that you recorded) and then tell it to write a line from there. Once you have written the new line, if it was at least as long as the old line, you are fine -- but if it was shorter than the previous line so that the overall file would be shorter, you have a problem. Windows and Linux and MacOS all define internal calls to allow you to "truncate" a file at the current position, but MATLAB does not provide any interface to such a function so you are stuck (and need to do something like write blanks to where the file used to end.) This all is a nuisance, so doing operations "in-place" in a file is usually only done for binary files, and for text files you typically end up reading everything, clobbering the old file, then writing all of the updated information.
