Legal Question in Landlord & Tenant Law in California

Your lease should have a provision regarding notices. Otherwise, regular mail should be fine.

In general, if notice by mail is sufficient it can be regular mail and does not have to be certified. it is a good idea, but not necessary, to ask them to acknowledge within 2-3 days that they received your notification and if they dispute it is legally proper, so that they do not later raise those issues [it does not legally prevent them but many people do not know that]. if they live reasonably close by, slip a copy under their door.