Do you mean "adoptive storage", aka "format as internal", or making it the default location for stuff that can be installed to a SD card formatted as portable (the old Android 2.2-style "move to SD")? These are quite different things.
For the first it's simple: if you have the option you can format it as internal (save your data first, obviously), and if the phone doesn't offer that then there's nothing you can do. Not all phones do allow this, and Samsung removed it from most models.
For the second you need to use a computer: install "ADB", enable usb debugging on the phone, connect via USB and use the command "adb shell pm set-install-location 2". It won't move anything you've already installed, you'd need to do that yourself. You can undo it by repeating the procedure but using "0" rather than "2" (from memory: it could be "1" - one of those sets it to "internal", the other is "auto", which will be your current setting).