npm install bootstrap
//OR
npm install bootstrap@latest
npm i bootstrap@4.6.0
or last version
npm i bootstrap@latest
npm install bootstrap@latest
npm install bootstrap@3.3.7 --save
npm install bootstrap@latest --save
npm install bootstrap@5.2.1
npm i bootstrap@5.0.0-alpha1
$ npm install bootstrap@3
npm install bootstrap@5.2.2