upgrade to 2.4

Run below command step by step to upgrade Magento 2.3.5 to Magento 2.4:

1) cp composer.json composer.json.bak (this command create backup of composer.json)

cp composer.json composer.json.bak

composer require magento/composer-root-update-plugin=~1.0 --no-update

composer require magento/product-community-edition=2.4.0 --no-update

Now open composer.json file and do following required changes in version manually: (a) "friendsofphp/php-cs-fixer": "~2.16.0" (b) "magento/magento2-functional-testing-framework": "^3.0" (c) "pdepend/pdepend": "~2.7.1" (d) "phpmd/phpmd": "^2.8.0" (e) "phpstan/phpstan": ">=0.12.3 <=0.12.23" (f) "phpunit/phpunit": "^9" (g) "sebastian/phpcpd": "~5.0.0" (h) "squizlabs/php_codesniffer": "~3.5.4"

(4) Last and final step run following command:

   composer update