A . If You updated from Typesetter 5.04 or earlier (?) :  Typesetter 5.1 is not really php7.4-ready .

( If You have clicked at the update-button in the old typesetter version : It has a different function.)

B. Instead of 5.1 please use the CE-Version 5.1.1 or the new version 5.2 (master).

:: To update to the CE-Version You must replace the /include-directory of the old installation with the /include-directory of the CE-version (make a backup before or rename it).

C. Please use filezilla-portable for the upload because several users had problems with other ftp-clients.

The update to TP5.2 is more complicated, because You also have to replace the gpconfig.php .

P.S. After successful update You should also  (only)  update the /addons-  and the /themes - directories!

Be careful not to overwrite templates which You use and in which You made specific code-adaptions.