Doc Library Expiration Date Automation

Greeting Everyone, 

I am in need of your expertise.  I created an expiration date field that automatically calculates 1 year from the date the file is uploaded.  I also created a flow to notify the document owner or the user who modified the document to review the document 15 days prior to the expiration date.   

However, I would like SP to automatically change the expiration date field when a user modifies the document and checks it back in.  How would I go about doing so? I am fairly new to SP Online and all functions previously mentioned were done by watching Youtube videos.  

Thank you in advance for any assistance anyone can provide.