how frequently can i change the sitemap ? now i changeing every 3month once can i change it weekly or montly,if i change at short time it will affect any of my SEO? please advice me
It depends on the type of website that you have, but generally I generate a sitemap every 2-3 days or so, to include the newly created links.
If your site is big enough and you have a huge amount of links I would probably create a cron job on your server to update the sitemap automatically everyday... Another option is to install an xml sitemap generator on your website... This software will allow you to run a new sitemap now and then..
The xml site map is intended to inform Google of new or changed pages and hence I would say that you only need to re-submit when you have new or changed pages.
Definitely depends on the frequency of web updates. If your website is fairly large and updates happen frequently, there is nothing harm in changing/updating a sitemap. It will do good than bad.
When you change the content on your site, you should update the sitemap. On my blog - its updated everytime I add or modify an article. On my forum - its updated every 24 hours. But both of my sites have automatic sitemap generators installed. Meaning I just add the content and the sitemaps take care of themselves.
You can update it any time when you update your internal pages of the website otherwise, there is no need of it.