It's in your user cp settings.
user cp -> Edit Options -> Thread View Options -> uncheck "display posts in classic mode"
What about for the whole forum?
admin panel >>
Configuration >>
Settings >>
Show Thread Options >>
Display posts using the classic layout <-- select
No
And see
common SQL queries & how to run them to run below SQL query
UPDATE `mybb_users` SET `classicpostbit` = '0' WHERE `classicpostbit` = '1'