Hi, I am trying to update my Macbook from Catalina to Big Sur, but it is impossible, how I can do it? please help!

What model is your MacBook? Look in About this Mac in the Apple menu.

Here are the requirements (along with instructions on how to upgrade).

Select all ofd the mailboxes in the sidebar and use the Messages ➙ Rebuild Mailbox menu item. Let it run for a bit and then click on the In box and see if the "unread" number is gone or not.

Too many windows/tabs open?