Changing your own articles
If you are the author or maintainer of an article, you may of course change it. Just one word of caution: After having worked on a specific problem for some time, it may sometimes appear that certain facts are ‘quite obvious’, thus deleting them from the article may help to ‘streamline’ the presentation. However, such intermediate steps may still prove useful to people who only start to work in the particular area.
Correcting and changing other authors' articles
How to proceed with changing other author's articles depends very much on the sort of change. Correcting a ‘slip of the pen’ should be unproblematic. In any other case, though, it is good practice to first discuss any changes with the article's author; in any case, the author whose work is changed should be notified by email.