You can change the citation style at any point, before or after citing.
How
Choose the style in the report toolbar. Every citation Silvertext wrote is rewritten in the new style: the in-text form, the quote marks and punctuation where the style differs, and the reference list entries and their order. A toast confirms the switch and offers Undo.
What is not changed
Citations you wrote yourself are not rewritten; they are checked against the new style instead, and any that no longer fit get a card with the corrected form. Your own reference-list entries stay as they are.
Accepted citation-format fixes
If you accepted a fix that corrected one of your citations, that fix follows the switch too, so the document stays consistent.
Choosing the right style
Use the style your course, journal or publisher asks for. If you are unsure, APA is common in the sciences and social sciences, MLA in the humanities, and Chicago in history and publishing.