If you want the link to appear on every page, consider adding it to the menu (Site admin > Appearance > Theme > Theme Settings)
If you just want it to appear on a single page, add the link to a content area (If you are familiar with html and your theme is a bootstrap one, you can use the html part of whichever editor to add the relevant bootstrap btn classes to style it)
If you want it on every page, but styled as a button, then you could either add it to your theme, or add it to an html block which can be added to every page.
Lots of ways - depends exactly what you want it to look like