If you have a Pro-Account or higher:
You can edit the path of your sitemap or you can add more sitemap.xml URLs when you are opening the "Crawler-Settings" on the top right of OnPage.org. Open the tab "What to crawl" and navigate to the "Sitemap URL". Here you can edit the current path or add new sitemap paths. Keep in mind that you can add more than one sitemap.xml.
Unfortunately, the sitemap cannot be changed within the FREE plan. We are crawling the sitemap.xml under the default path www.domain.com/sitemap.xml.
If you want to change the sitemap.xml path or add more than one sitemap.xml you should consider to upgrade to the PRO plan.
Here is an interesting link about Common SEO Mistakes and How You Can Fix Them.