If you don't have one, or don't know where it is, you can find it by visiting this page: https://YOURTENANT-admin.sharepoint.com/_layouts/15/online/TenantAdminApps.aspx
Select Apps for SharePoint:
Upload the .sppkg file to the App library. You may optionally tick "Make this solution available to all sites in the tenant".
Click Deploy.
The solution will be deployed to your tenancy:
Install to a Site
You can skip this step if you selected "Make this solution available to all sites in the organization" above.
Now that you've uploaded the app package, you can install it to a SharePoint site.
Navigate to your site and click on Site Contents. Select New > App: