etran to find out the version you need, you can look for example on Packagist https://packagist.org/packages/fof/terms and check under the "Requires" section which versions of Flarum a particular version will work.
Using fof/terms as an example, the latest version compatible with beta 13 would be 0.4.3.
You can then use composer require fof/terms:0.4.3
If you do this, the version will be hard-coded in your composer.json and composer update commands will never try to update it.
To get back to the very latest version and remove the hard-code, you can run composer require fof/terms without any version. Or you could edit composer.json to change the requirement to something different.
