Can I update a GPL version to premium version. using product key

Hi MD. Ibnul,

Once you’ve purchased your GeneratePress Premium subscription, you may download a copy of GP Premium through this link: https://generatepress.com/account/

Since GP Premium is an extension plugin for GeneratePress theme, you’ll need upload your GP Premium download via Plugins > Add new > Upload Plugin.

See this for reference: https://share.getcloudapp.com/YEuBW6AN

Then, you may simply add your License Key through Appearance > GeneratePress

Here is a document you may refer to for more assistance regarding this: https://docs.generatepress.com/article/installing-gp-premium/

Hope this answers your question! :)