I do not understand what it is you are referring to here. You can't change the amount of storage space a device has. I am going to guess that you really mean you want to remove things from the device to free up storage space?
If that is the case, then there is only so much you can do. The OS and many preinstalled apps will be taking up a lot of that space. You can uninstall some apps. Either ones you installed yourself, or ones that were preinstalled, are not necessary, and will let you uninstall them. With apps that you can't remove, but are updated, you can remove the updates for them to free up some space. But all that is limited.
If you want to just move files (photos, videos, music, etc.), not apps, to the SD card you can use the file manager app that is on the device, or if there isn't one you can download and install one to access and move the files. Sadly, they usually only let you move one or two at a time.
To move them all at once, or in large amounts, you can connect the phone to a computer via USB cord, then move the files over to an empty folder on the computer. Once there, and still connected via USB, you can then move or copy the files over to the SD card. Personally I would suggest leaving a copy on the computer as a back up.
Once that is done you should then go into the apps that you access the files through (like gallery, etc.) and change the apps individual settings to look for the files on the SD card. You may also want to change the settings in apps that you use to create new files (like the camera) to save new ones to the card rather than the phone.
Now if you are wanting to move apps to the card, that is a whole other story.