Go to the apps catalog page from the below URL format and click on classic experience options.
- Click on App Catalog as shown in below.
- Click on App for SharePoint as shown in below.
- This displays the Apps for SharePoint page.
- Click on upload, choose your file and upload the file.
First open your site collection where you need to install your app, click on site content and then click on Settings gear icon.
- You can either click on Add an app or App under New.
- This displays the SharePoint apps page.
- You will navigate to the page where ADD is visible as shown in below, click on it.
- And finally click on Trust it, application starts installing.
- Once application got installed you can check in Site contents.