One way of doing it is: at the bottom, below all your other stuff, of your .htaccess file in the root of your WordPress installation you can add the following:
Redirect permanent /path/to/old/post/old_post_name.html https://www.yourdomain.com/path/to/new/post/new_post_name.html
And change the url path, post name, and domain name to what you need.
Please note that everything has to be on the same line (no carriage returns).
To learn more you can do an internet search on:
Redirect permanent
Redirect permanent .htaccess
Mod_rewrite
Mod_rewrite .htaccess
The different options depend on whether you can configure the server yourself, or a third party does this. Use .htaccess in this case for example when you are using shared webhosting. Be aware that it can pretty technical stuff, in the beginning. Once you know your way around it gets a bit easier, usually.
You can leave the code there until everybody has updated their url path. For example search engines. AFAIK having lots of rewriting can cause slow down for a website. So optimally I remove them after everything has updated on the internet.
Please note: I am not sure if technically speaking the code has to be at the absolute bottom of the .htaccess file after all the other stuff in the .htaccess file, that is just the way I learned it.
If you’re not sure what you are doing ask someone before implementing.
