I started CS50's Introduction to Computer Science course 26th of March 2020. I have not a clear idea about assignment deadlines and course end date. I want to take the official certificate after completing the course. But I can't find the deadline for upgrade verification.
Deadlines are explained here: https://cs50.harvard.edu/x/2020/faqs/
CS50x does not have individual deadlines for problem sets or for the final project. You are welcome to work on and submit at your own pace. If all required work is turned in and received a satisfactory grade or before 31 December 2020, you will be considered to have completed CS50x 2020.
There's also a section explaining what steps are possible if one does not complete by the end of 2020.