The Wiktionary community encourages users to be bold in updating entries. Wikis develop faster when people fix problems, correct grammar, add facts, make sure the language is precise, ensure conformance with our policies and consensuses (especially our criteria for inclusion and entry layout standards), and so on. Feel free to update entries as you see fit, but be prepared that these changes may also be overwritten and in some cases, removed straight away.
New users in particular are often entranced by the openness of Wiktionary and dive right in. That's a good thing. But please note: "be bold in updating pages" does not mean that you should make large changes or deletions to long, complex entries that are well established and structurally sound, such as lead, or to policy pages such as Wiktionary:Criteria for inclusion. In such cases the text as you find it has come into being after long and arduous negotiations between Wiktionarians of diverse backgrounds and points of view.
If you feel there may be disagreement with your edits, it's a good idea to discuss the entry on one of our discussion rooms. Then wait for responses for at least a day: many people edit Wiktionary in their spare time and may not respond immediately. If no one objects, proceed, but consider moving large deletions to the talk page and listing your objections to the text so that other people will understand your changes and will be able to follow the history of the page. Also be sure to leave a descriptive edit summary detailing your change and reasoning.
With the vast majority of entries, feel free to dive right in and make broad changes as you see fit. It's only with a few sensitive pages that caution is better advised, and you'll probably recognize those right away. Even if you don't, as long as you have an appetite for debate, being bold is generally a defensible position. You won't be the first person to have made a controversial edit, and you won't be the last. That said, contributions that add new definitions or example sentences to an entry are likely to be more welcome than contributions that just delete some of the content. Just remember, be bold.
Some caution is also advised if your changes affect many other pages, such as editing a template or moving a highly linked-to page (you can always check what links to a page before moving it). While not required, it is recommended that before making this type of major change you become more familiar with the relevant policies of Wiktionary. Also, it is considered polite if you are willing to fix any problems you have created (such as broken redirects or formatting problems) in the affected pages.