You can suggest edits to posts on the main ELU site, which will be approved or rejected by users with higher reputation.
Or, once you've crossed a certain reputation (say, 2000), the system grants you extra privileges, one of which is that you'll be able to make edits to any post you see, and it will be approved instantly without peer review.
That's why we have these things like reputation and badges and stuff.
Regarding leaving comments with suggestions, it's actually the very reason we have the comments section.
Leave constructive criticism and suggestions for improvement wherever you find mistakes, and remember to be nice, and avoid posting comments targeting the user instead of their post.
Regarding the privileges to suggest edits or make instant edits on meta, the rules may be a bit different, and probably because meta is mostly for discussion, where users frequently post their own opinions regarding several issues in order to reach a community consensus, and if other users edit these posts, it could mean losing key information. So, unless a meta post is marked community wiki, or unless you have a certain reputation and the associated new privileges, you cannot make changes to other users' meta posts.
All the rules and FAQs are available in the help center.