How can I reload .emacs after changing it?

Very strange that the very convenient

M-x eval-buffer

is not mentioned here.

It immediately evaluates all code in the buffer, its the quickest method, if your .emacs is idempotent.

You can use the command load-file (M-x load-file, then press return twice to accept the default filename, which is the current file being edited).

You can also just move the point to the end of any sexp and press C-xC-e to execute just that sexp. Usually it's not necessary to reload the whole file if you're just changing a line or two.