We'll assume that you're using a host which provides CPanel. Most do. If not, then installation is also possible by other methods such as FTP.
Open your browser and enter www.yourwebsite.com/cpanel into the top URL bar. This will ask for an accountname and password, which were provided by your hosting company. Once logged in you should see the CPanel main page as shown in the adjacent image.
Find and click the 'File Manager' icon. There may be a question about where you want to go with File Manager. Just accepting the defaults is generally fine.