To modify an A record, just enter the desired IP address in the field and click Change.
To modify an MX record, just enter the desired mail server address in the corresponding field and click Change.
Make sure you have set the correct domain/subdomain name in the host settings for your IP address. Otherwise the domain/subdomain will not work.
In case you'd like to reset an A record to the default IP address of our server or an MX record to the default mail server address of our server, just hit the Reset button, corresponding to this record.