Assuming you're using the the visual editor (tabs at the right hand side
switch between visual and plain text) you can use the link editor from
the
button bar above - it's the one that's meant to be three links of chain.
Hover
over the buttons an you should get tooltips to tell you what they do.
I've
circled the button in the attached image. If you use the text mode it's
just
html if that's any use to you.
If you search for 'wordpress editor introduction -gutenberg' you should
find
some documentation. Gutenberg is the name of the new version of the
editor,
but that's not on our site yet.