First thing you need to do is to find what view (page) you want to modify. It is useful to activate an option that shows path of views as comments in code. You need to active plugin "Elgg developer tools" and activate option "wrap views" (or something like that).

Now you can see in html code path of view you want to modify, you have to write a plugin and overwrite that view.

Create a plugin: http://docs.elgg.org/wiki/Plugin_development

Views functions: http://docs.elgg.org/wiki/Engine/Views