Go to your composer.json file
And set below :
"require": {
"php": ">=7.0.0",
"fideloper/proxy": "~3.3",
"guzzlehttp/guzzle": "^6.3",
"laravel/framework": "5.5.*",
"laravel/passport": "~4.0",
"laravel/socialite": "^3.0",
"laravel/tinker": "~1.0",
},
And then run composer update
command.