THOU SHALT NOT
Stack Exchange, in general, thrives on community members ruthlessly editing questions and answers to improve them. If a question is clumsily worded or is too long or has too much extraneous information, it's ripe for editing.
Worldbuilding, in specific, is quite different. We do not really tolerate much editing of other people's questions or answers. Except for certain well delineated circumstances. Anything you're not sure about is best addressed in comments, especially if you feel the need to edit content rather than mechanics.
Things you can not do include:
- excising blocks of text that you feel don't belong
- rewrite the question so that it conforms to your own prose style
- change a critical fact or aspect of the question
- remove any critical background information or presume to add something
- remove any of the querent's stated goals, objectives or considerations
- in general, presume that you know the querent's needs better than she does and therefore decide to "improve" her query by making it your own
- take over an old question
That said, there are still plenty of things you can do! And these are just as important.
You can always edit an incorrect real world fact. Be very careful that the querent isn't couching the question in terms of fictional world fact!
You can always and freely edit spelling, grammatical or mechanical errors. Be ware: we accept both British and American standard spelling conventions. You can always add standard units when only metric is given, and you can always add metric when only standard units are given. If you feel up to scaling a Wall-o-Text, you can break that down into paragraphs.
You can improve a query's format for clarity and add logical section headers.
If you are sufficiently competent with a foreign language, you can translate a question or answer written in any language other than English.
You can edit a non English speaker's grammar and diction with the caveat that you do not change what they intended to say. This is important, because the question, while potentially universally applicable, is also specific to the querent's needs.
You can always edit out what I'd characterise as "snark". If you see a paragraph that says something like "Some dip$hit closed my question so now I'm forced to edit..." that can be deleted.