When DNS records have been changed it takes about 24 hours for the changes to propagate and take effect. Only after that you will be able to see the result of the DNS changes you have made.
If after 24 hours the DNS change has not taken effect yet, try clearing your browser's cache.
To resolve a domain name to a certain IP address without having to wait for the DNS change to propagate, you can modify your hosts file.
Important! When you change the nameservers of a domain, all advanced records (A-records, CNAME-records, MX-records, SRV-records and so on) would resolve from the DNS zone of the provider, whose nameservers you are setting. Thus, if you are using a custom A-record or MX-record for your needs, it is strongly recommended to add them in the DNS zone of your new provider in order to ensure minimal downtime and / or propagation.