Research and Development
Bash shell uses a startup file to set up the environment. The file determines certain Bash shell configurations for the shell itself and system users.
To complement and contrast the two most popular answers, . ~/.bashrc and exec bash. Both solutions effectively reload ~/.bashrc, but there are differences:
. ~/.bashrc or source ~/.bashrc will preserve your current shell
Except for the modifications that reloading ~/.bashrc into the current shell (sourcing) makes, the current shell and its state are preserved, which includes environment variables, shell variables, shell options, shell functions, and command history.
exec bash, or more robustly exec "$BASH"[1] will replace your current shell with a new instance, and therefore only preserve your current shell's environment variables (including ones you've defined ad-hoc)
In other words, any ad-hoc changes to the current shell in terms of shell variables, shell functions, shell options, command history are lost. Depending on your needs, one or the other approach may be preferred.