Hello Bas60
No, you don't have to download the 7.5 ISO (but you may want to for any future clean install)
Wait a day - my system often reports by email that an update is available but the Software installer doesn't catch up until hours later. I assume there is a cron job running a script that generates the email and it runs before the cron job that lets Software installer know.
If it still doesn't catch up, logon to the server (either directly or by using PuTTY) and enter the following
yum clean all
yum update
The first command tidies up the yum repositories, in case there is a problem.
The second command runs the yum update process.
This should update your system from 7.4 to 7.5 - it just worked with mine.
Cheers
Ian