There are tons of good proofreading courses available online. Udemy, Skillshare, and LinkedIn Learning offer excellent copyediting and proofreading micro-courses that you can complete relatively quickly. If you want to self-edit your work, you can check out How to Find & Correct Writing Errors (Udemy). To become a professional proofreader, you can check out Knowadays, Universal Class, The Publishing Training Centre (UK), The Chartered Institute of Editing and Proofreading (UK), Scribendi, and Doris & Bertie. These classes range in price.