How to downgrade angular7 to angular6

You need to set the version numbers in your package.json for (at least) this packages

"@angular/animations": "^7.0.0",
"@angular/cdk": "7.3.3",
"@angular/common": "^7.0.0",
"@angular/compiler": "7.2.6",
"@angular/core": "7.2.6",
"@angular/forms": "^7.0.0",
"@angular/http": "^7.0.0",
"@angular/material": "7.3.3",
"@angular/material-moment-adapter": "^7.3.3",
"@angular/platform-browser": "^7.0.0",
"@angular/platform-browser-dynamic": "^7.0.0",
"@angular/router": "^7.0.0",
"rxjs": "6.4.0",

as well as in dev-dependencies

"@angular/compiler-cli": "^7.2.6",
"@angular-devkit/build-angular": "0.13.3",
"typescript": "3.2.4",
"@angular/cli": "~7.3.3",

Also make sure to downgrade every library that needs to be downgraded depending on angular's version. Check error messages after step 1...

Once you changed that, run

rm -rf node_modules

from the project's root folder to remove all packages

Then run

npm i 

And you should be good to go