Older versions of ESXi are still amazing pieces of server software that were made available for free back in the days, but today there is no official way to get new licences from VMware. So here is a neat trick how to do it through the console (you need to have the SSH access/tech support mode enabled):

ESXi 3.5 / 4.x

rm /etc/vmware/vmware.lic
rm /etc/vmware/license.cfg

services.sh restart

You can also add first two lines to your /etc/rc.local file so your evaluation period gets restored every time you reboot the physical host machine.

Bonus: sometimes it’s easier to just enter licence, here is one you can use:
HH09H-6A015-N6000-0RCY6-CPZQH

ESXi 5.x

rm -r /etc/vmware/vmware.lic
cp /etc/vmware/.#license.cfg /etc/vmware/license.cfg

/etc/init.d/vpxa restart
services.sh restart

Your evaluation licence should now be reset for another 60 days.