If you wish to change the negative balance limit for an individual member, use Tool #20 "Update Account Information."  (This can also be done while in Tool #982 "Work Collections" when working your negative balance accounts.)
If you are granting an automatic negative balance limit for all members when they open an account, you may wish to review the default negative balance limit in the share dividend application configuration (Tool #777 "Savings/Checking Products Configuration").  This is also where automated negative balance processing limits are set (using the F20-ANR Scoring feature).  So if your credit union automatically assigns limits using the monthly scoring feature, you may need to review those limits as well.