Download the CRX file to your computer for the Chrome extension that you want to install if available/some website provide a link to download .Zip file.

Go to chrome://extensions/ and check the box for Developer mode in the top right.

Before check Developer Mode

After Check Developer Mode yo will see like below image.

if you have CRX file then use a CRX Extractor app – I used CRX Extractor – to unpack the CRX file and turn it into a ZIP file.

Locate where the ZIP file on your computer and unzip it with WinRAR or etc.

Now go back to the chrome://extensions/ page and click the Load unpacked extension button and select the unzipped folder for your extension to install it.

If you want to upload/install a pack extension click the pack extension button and upload like CRX file.