Login to Admin Mode, go to the page which you want to change, and hover over its "Page:" link near the top of the page. This will cause the Page Menu to be displayed. Click the "Edit this Page" link on the Page Menu.

Scroll past the first few fields on this form. The Meta Tags are located near the bottom of this form.

Meta Description: This should be a long description in sentence format that describes what the user can find on this page. It can be one or a few sentences long. Search engines use the Meta Description field in various ways, but if it contains a representative description of the page's content then it can be beneficial to the page's search engine engagement if it helps users find the content they are looking for. You should try to have a different Meta Description for each page on your site.

Meta Keywords: This field is optional and if it is populated it should contain a list of keywords that users might use in search engines to locate this page. Most search engines ignore this meta tag therefore there isn't much if any benefit in populating this list of keywords.

Click the [Save Changes] button.