Many businesses see their traffic drop after launching a new website. The design is better, but Google can no longer find the pages it knew. The fix is simple if you plan it before launch.
Keep the same addresses where you can
If a page ranked at a certain address, keep that address on the new site. It is the safest option.
Redirect every address that changes
When an address must change, set a permanent (301) redirect from the old address to the new one. This passes most of the ranking to the new page.
Carry over titles and descriptions
Copy the meta titles, descriptions and headings of your best pages, then improve them. Do not launch with empty SEO fields.
Submit the new sitemap
After launch, submit your sitemap in Google Search Console and watch the coverage report for errors over the next few weeks.