There is a GUI called "Firmware", but i never use it.
Pretty self explanatory terminal commands:
fwupdmgr refresh
fwupdmgr get-updates
fwupdmgr update
If you have a laptop you might have to raise the lid to get all the updates.
It works pretty well but, as Harvey mentions, it doesn't have everything. For example, it will do my Lenovo X1 Carbon Gen 8 but not my Thinkpad Pro Dock.
I notice fwupd has been updated. It wasn't working when I installed Solus 4.4 just before 4.5 came out. Fortunately, the snap works fine.