There are quite a few factors that determines the approach such as what type of version control are used, what is the size of the change, what is the impact of the change, is it a new procedure or replacing an existing and so on.

If it is a new, then just drop the wrong one

if it is a replacement then how big is the change and what will be the possible impact, depending upon you can have the entire database backed up or just create a script for your original procedure before messing it up or you just do an ed and change the file back to original and reapply. you may rename the old procedure as old and then work on new and so on.

few issues to keep in mind are synonyms, dependancies, grants, any job calling the procedure at the time of change and so on. In nutshell, scenario can be varied and solution also can be varied.