This article is written assuming that you have already logged in to your account. 

• In the dashboard, go to the 'edit check' option of the check.
• You'll find the 'apdex response threshold' set as 750 ms. 
• Change the 'apdex response threshold' to any desired value between 0 ms to 1000 ms.
• Click on 'save change'.

'Apdex response threshold' for that check will be updated. Not that you cannot change the 'apdex response threshold' for all the checks at a time. You'll have to do it individually for every check.