Expanding a little bit on what bakunin has already said...
If a script's first line is:

Code:

#!/usr/bin/ksh93

and that script is made executable such as by:

Code:

chmod +x script_pathname

then whenever you run that script by invoking its name, it will be run by ksh93. Whether or not you do that, if you issue the command:

Code:

ksh93 script_pathname argument...

that will run script_pathname with the given command line arguments using ksh93 to interpret script_pathname no matter what any #! on the first line in that file might request be used as its interpreter as long as /usr/bin is in your PATH environment variable.

If you issue the command:

Code:

exec ksh93

your currently running shell will be replaced by an invocation of ksh93 and it will be used to interpret any interactive commands you issue in that terminal session until terminate that shell.

If you issue the command:

Code:

ksh93

and /usr/bin is in your PATH environment variable, you will start another shell running in the current shell execution environment and it will be used to interpret any interactive commands you issue until you terminate that shell (and then you'll resume the previous shell session.

If you issue the command:

Code:

chsh /usr/bin/ksh93

and it works, then the next time you login to your system, your login shell will be ksh93
and it will remain in place as your login shell on that system until you run chsh again.