Some controversial or vandalism-prone articles are locked and require accounts to edit (you'll see padlock icons at the top right of the page in their cases). But accounts are unnecessary to edit 99% of Wikipedia's articles. So if you or someone else see bad information somewhere, just click the section's "[edit]" link (or the master "Edit" link at the top of the page) and make your changes. I do this all the time without hesitation -- at least when it comes to subjects where I actually possess some facts.
I regret that Wikipedia has gotten this reputation. It is undeserved and comes exclusively from people abusing its design, i.e. the intent of its creators. The main means is what Mr. Richard cited (people not adding citations). But the other big one is bias -- i.e. citing sources while still lying through omission. Thankfully, that's only typical when it comes to political articles. Otherwise, the site generally is not flawed in its design. It provides marvelous, full facilities for citing up to literally every sentence in an article, or even every implication within a single sentence, if necessary. And as long as people make use of that, the site works well as as an invaluable source of verifiable knowledge ... well, again, when it comes to non-political articles, at least.
Anyway, don't be afraid to edit the site without an account, folks. They don't mind. Just cite and all will be fine. There are all sorts of automatic alerting mechanisms built in so that various moderators learn about and review anonymous changes. If you do anything wrong by accident, it will usually be undone very quickly and as long as you're still at the same IP you had when you made the unwanted change, a little notice will appear at the top of each page on the site when you next visit, letting you know, so you can go back and try your changes again properly.