There are few ways to do this. First Log in to SharePoint – How to login to SharePoint

In order to upload a document you need to have at least Edit, Contribute, or Add and View Permissions. If you don’t have that permission, talk your site administrator in order to the get right permission.

Go to the SharePoint Site and click the library where you want to upload your document.

There are the three main ways to upload a document to SharePoint

01. Drag the file. 

02. Click on the new document 

Then click “UPLOAD EXISTING FILE” Click browse and select your file Click OK

03. Upload Document 

Click on the library where you are going upload the file. Click FILES on top left in the ribbon bar. Click Upload the Document

Then click Upload Document – Click “Browse”, select your file, and click OK.