Craft 3 CMS Update Error
Try replacing your "require" section with this in your composer.json file:

"require": {
    "craftcms/cms": "^3.0.18",
    "vlucas/phpdotenv": "^v2.4.0",
    "craftcms/redactor": "^2.1.5",    
    "dolphiq/craft3-forms": "^1.1.0",
    "craftcms/contact-form": "^2.2.2",
    "craftcms/guest-entries": "^2.1.3",
    "nerds-and-company/schematic": "^4.0.14"
  },


Then, if you have Composer installed from the command line, run composer update from your project's root and let it finish.