For best accessibility practices, follow these simple rules:
A link (navigation) should look like a link and be coded like a link.
A button (action) should look like a button and be coded like a button.
However, there are of course, exceptions to the rule:
If you are using a <a>-tag for an action (as in submit form, even if it takes you to a new URL), change it to a <button>
If you are using a <button> to change URL (location), change it to a <a>.
However, if it is the action that is changing the URL, keep the <button>
If the URL is not reachable by direct linking/deep linking, keep the <button>