When you update the nameservers for a domain, it may take up to 24-72 hours for the change to take effect. This period is called DNS propagation.

In other words, it is a period of time ISP (Internet service provider) nodes across the world take to update their caches with the new DNS information of your domain.

Due to DNS caches of different levels, after the nameservers change, some of your visitors might still be directed to your old server for some time, whereas others can see the website from the new server shortly after the change.