An internal link is a link embedded in the text of one web page that links to another page on the site.
To create an internal link, highlight the text within the page which is to serve as link text then select the "insert/edit link" button).

In the "Insert/Edit Link" dialogue window, click on the Internal radio button and then the Browse button.

Browse through the site to select the desired file.

Confirm the page selection by selecting the confirm button.
The "Insert/Edit Link" window will reflect the page selection. Select the Insert button.

As always, remember to submit the page.