This is a very good question.
The answer to the referenced post requires a change to the WP options. They are stored in the database in the
wp_options table (assuming
wp_ is your prefix).
There are a few ways to change these in this case:
- Direct DB access: if you're comfortable modifying the WP DB directly (and you have access) you can add a record to the table. Many (or all) of the tables in the WP database should be prefixed the same,
wp_ by default, although that default is often changed. That is your table prefix. Find the table that looks like
wp_options if your prefix is
wp_) and add a record to it. If you're using phpMyAdmin or a similar tool, you'll only need to know the key and value to enter. Use the key
upload_path and put in some non-empty value. This should make both fields show up in the admin UI. You can do the same with a SQL insert statement.
- WP CLI: this is the method mentioned in the answer to your referenced post. If you've not used the CLI (command line interface) before, it's essentially a script shell for changing WP things. This wraps the DB access for you in this case. Setting it up usually isn't difficult, see here for how.
- A Plugin: I haven't seen a particular plugin for changing WP options but I would assume that there are probably several. You could add this to an existing plugin on the site by adding the line
update_option('upload_path', '/path/to/uploads');. That code could go anywhere, including in a theme. Once run, you should remove it (or you'll never be able to change this value to any other option).
Warnings: you are changing a WP option. While the effects of this option are limited to uploading (and related), there are things in the option table that, if modified incorrectly, will completely torch your site. If you're in a development environment that you can restore fairly easily, hack away. Be very careful doing this on a production site, with care including full verified backups before changing anything. Things like this are always best tested in a non-production environment first.
Either way, you will need to know the full canonical path to your upload folder and you'll need it to be set with appropriate permissions. Exactly what those are depend on your system. There are plenty of docs on the Internet which explain how to figure out those details correctly.
Also bear in mind that (at least according to the referenced answer) this option is set to be deprecated in some future WP version. I haven't checked that so I don't know if it's true in > 4.9.8 but it appears that the option header is still there on my 5.1.1 installation (once again, not tested). I also don't know what these options will be replaced with once they are removed. One would assume that the ability to upload to a non-default location wouldn't be removed, although how uploads work may change or the admin UI for this part of the WP world may be overhauled, either for some number of reasons.
Please ask if you need more detail on a specific part of this or if something is unclear.