I was using a 16 GB micro sd card with raspberry pi4. Now I need to expand my memory . This is why I want to move the installed raspbian from 16 gb to 32 gb. How am I supposed to do that?
Raspbian includes a "SD Card Copier" on the tools menu. That will copy just the used part of your 16GB SDCard to your 32GB SDCard and give you a bootable clone.
It's been there since 2016 and there's some docs at: https://www.raspberrypi.org/blog/another-update-raspbian/
Make an image of your SD card you want to expand. This will make an ISO file that is a clone of that disk.
Insert the blank 32gb SD into a reader, and select in the baker program, and then it will write the image of the 16GB card to the new 32GB card.
You can keep the 16GB for backup or repurpose it.
If you like the Linux command line version, look up a duplicate post of how to run a 'dd' command line... or use one of the GUI based SD cloning tools.
It's a little tricky at first, but try not to mix up your disks, as you don't want to accidentally make your 16GB a target. dd might let you do directly though, so you won't need to dump the image to another storage. You might find a GUI program that has this function.