Updating ESXi, vSphere general questions
Solution 1:
You should be able to find ESXi iso on the VMware website: https://my.vmware.com/group/vmware/downloads/details?downloadGroup=OEM-ESXI70GA-DELLEMC&productId=974 You can easily upgrade your ESXi installation using the iso. Just mount it to your Dell server and follow the guide below: https://www.starwindsoftware.com/blog/how-to-upgrade-esxi-6-7-to-7-0-without-vcenter The update sequence is right, I would add one step to make backups before doing the upgrade.
Solution 2:
From general to the specific :
Keep in mind to always check if the ESXi version you want to install is allowed on your hardware parts : https://www.vmware.com/resources/compatibility/search.php
Have a double check if your ESXi version is compliant with your current vCenter, else you should update your vCenter a different version. Check here : https://www.vmware.com/resources/compatibility/sim/interop_matrix.php#upgrade
Here's a link to check if you can upgrade from X.X ESXi version Y.Y version https://www.vmware.com/resources/compatibility/sim/interop_matrix.php#upgrade
Once you're sure about these 3 points, you can update your ESXi safely. Once it's done, upgrade VMTools + VM Compatibility (often forgotten..) + VMFS version from your datastores (depending from which version you come from)
If you want to gain time when you update your ESXi, avoid to create a usb key then reboot your server. You can download .zip files from your provider (dell custom esxi offline bundle in google for instance) then use this command from SSH command line to update/upgrade and reboot your ESXi : https://kb.vmware.com/s/article/2008939
Good luck :)