How to upgrade to Drupals latest version via Composer? Currently I have 8.6.13 installed.

composer update drupal/core --with-dependencies
Package "drupal/core" listed for update is not installed. Ignoring.
Loading composer repositories with package information
Updating dependencies (including require-dev)
Nothing to install or update


When I tried to look in to composer.json we don't have drupal/core in 

require.

https://www.drupal.org/docs/8/update/update-core-via-composer

How to upgrade to 8.7.0?

$ composer update
Loading composer repositories with package information
Updating dependencies (including require-dev)
Your requirements could not be resolved to an installable set of packages.

  Problem 1
    - Can only install one of: egulias/email-validator[2.0.x-dev, 1.2.0].
    - Can only install one of: egulias/email-validator[2.0.x-dev, 1.2.1].
    - Can only install one of: egulias/email-validator[2.0.x-dev, 1.2.10].
    - Can only install one of: egulias/email-validator[2.0.x-dev, 1.2.11].
    - Can only install one of: egulias/email-validator[2.0.x-dev, 1.2.12].
    - Can only install one of: egulias/email-validator[2.0.x-dev, 1.2.13].
    - Can only install one of: egulias/email-validator[2.0.x-dev, 1.2.14].
    - Can only install one of: egulias/email-validator[2.0.x-dev, 1.2.15].
    - Can only install one of: egulias/email-validator[2.0.x-dev, 1.2.2].
    - Can only install one of: egulias/email-validator[2.0.x-dev, 1.2.3].
    - Can only install one of: egulias/email-validator[2.0.x-dev, 1.2.4].
    - Can only install one of: egulias/email-validator[2.0.x-dev, 1.2.5].
    - Can only install one of: egulias/email-validator[2.0.x-dev, 1.2.6].
    - Can only install one of: egulias/email-validator[2.0.x-dev, 1.2.7].
    - Can only install one of: egulias/email-validator[2.0.x-dev, 1.2.8].
    - Can only install one of: egulias/email-validator[2.0.x-dev, 1.2.9].
    - Can only install one of: egulias/email-validator[2.0.x-dev, 1.2.x-dev].
    - Can only install one of: egulias/email-validator[2.0.x-dev, 1.2.15].
    - drupal/core 8.7.0 requires egulias/email-validator ^2.0 -&gt; satisfiable by egulias/email-validator[2.0.x-dev].
    - Installation request for drupal/core 8.7.0 -&gt; satisfiable by drupal/core[8.7.0].
    - Installation request for egulias/email-validator ^1.2 -&gt; satisfiable by egulias/email-validator[1.2.0, 1.2.1, 1.2.10, 1.2.11, 1.2.12, 1.2.13, 1.2.14, 1.2.15, 1.2.2, 1.2.3, 1.2.4, 1.2.5, 1.2.6, 1.2.7, 1.2.8, 1.2.9, 1.2.x-dev].
I had exactly the same issue while performing upgrades. The solution for me was removing/renaming the vendor folder and the composer.lock file and run composer update drupal/core --with-dependencies.

You can also try to update dependency explicitly with composer update egulias/email-validator drupal/core --with-dependencies. This usually works in this type of issues, but it didn't work for me.

Given that you don't see drupal/core in your composer.json you might also try composer require drupal/core.