How do I… · Blender
Adding a modifier is non-destructive — the mesh underneath is untouched. Applying one bakes it in permanently, and the order in the stack matters.
You must be in Object Mode to apply — in Edit Mode the option is greyed out, which is the most common reason it seems missing. Applying a Mirror before you have finished modelling means mirroring by hand from then on, so leave it until last. Objects with shape keys refuse to apply most modifiers at all.
This page is written by the people who make Open Portal: press Ctrl+Shift+O, box any part of your screen, and ask about it. Blender included — it works in every app, because it reads pixels rather than plugging into anything.
Faster than searching: box the part of your screen you cannot work out and ask about it directly — in Blender or anywhere else. Answers arrive where you drew the box.