I renamed my form but when i share it on facebook , the old title still

I renamed my form but when i share it on facebook , the old title still appears instead of the new one ! what should i do please ?

Facebook saves the information of shared links, this helps them to quickly load the information without needing to refresh it each time a link is shared. 

However, you could update the information Facebook has using their Sharing Debugger tool, it can be found here: https://developers.facebook.com/tools/debug/ 

Enter your form's link and click on the debug button, once you get the current information, click on the "Scrape Again" button so the new information is displayed. 

I hope this helps.