A .CSV file makes sense in a directory under the /root file system. If your user profile name is CKW, you might, for example, have an assigned home directory named /home/CKW. Uploading into that directory would be a reasonable thing to do.
You might use FTP to connect, change your target directory to /home/CKW, and issue the FTP PUT command to upload the .CSV.
Or your home directory might be shared out the the Windows network. If your AS/400 is known as MYAS400 in your Windows network and your home directory is shared as the name CKW, you might simply drag and drop the .CSV into the share in Windows Explorer. You would see MYAS400 as one of the listed systems in the Network neighborhood. You’d expand it just like you would for any PC that shows up for your network, and you’d see the CKW shared directory.
Or you might have some other connection that we don’t know about. Perhaps your AS/400 has access to shares on your PCs and you don’t need to upload at all. Just put your .CSV into a network share.
Once the .CSV is seen by your AS/400, either because it was uploaded or because its location can be accessed by your AS/400, you would use the CPYFRMIMPF command to import it into an existing database file. (It is possible to cause a database file to be created, but some preparation is necessary.)
Since we have no idea how your AS/400 and your Windows networking have been configured, we don’t know what you might be able to do.
Provide us with your AS/400 OS version/release, your PC OS version, and possibly an outline of how your AS/400 connects with your local network. We might then have a significant chance directly helping.