How do I get to the next search/find match result, or perform a search/replace?

The Overleaf editor has a Find menu in the top bar which offers a few options, e.g. Find, Find next/previous, Replace, Replace all, etc. After typing in a search expression and hit enter, the first search result will be highlighted. You can then jump to the next search result using the keyboard shortcut Ctrl-G.

To perform a Replace (All) operation, first type the search expression and hit Enter, and then type the replacement string and hit Enter. Here's a quick demonstration:

Regular expressions work, too: just put / around your search expression. You can then refer to capturing groups by $1, $2, etc.

These functions can also be accessed via keyboard shortcuts — you can find a list of all shortcuts available on Overleaf here.