There are a few options.
Develop locally
You can use a local server on your computer (I prefer using Desktop Server). You can import your current live site into Desktop Server, then make all the changes you want. When you have finished making changes, you can easily deploy the local version of your site to a live server.
Staging environment on your host
Some WordPress hosts offer staging environments with their hosting plans. This lets you click a button and create a copy of your site in a few seconds. Make all the changes you want on the staging site, then click another button to merge the changes back to your live site. I know WPEngine offers this, but I’m sure others offer a similar option.
Create a duplicate site
Similar to the first two options, you could always create your own staging environment online. Use a subdomain from your existing domain (https://dev.yoursite.com), or set up an entirely new domain to test on (https://yoursitedev.com). You’ll be using a separate database and set of files so no changes will touch the live site until you merge them. You can use a plugin like WP Migrate DB or a number of other similar plugins to export your database from one site, then import it to the other. If your development site is on the same hosting account you can try to copy changed files from one site to the other. When using this option, be sure to check the “Discourage search engines from indexing this site” box in your development site’s settings. You can even use some htaccess coding to block visitors to the development site without a valid password.
