FortiGate Next Generation Firewall utilizes purpose-built security processors and threat intelligence security services from FortiGuard labs to deliver top-rated protection and high performance, including encrypted traffic.
Article Id 191407
This article describes how to reset web filter FortiGuard quota.

Web filter profile category usage quota is configured to limit users from accessing bandwidth consuming web sites to specific amount of time or data package. 
A quota will allow access for a specified allocated traffic, calculated separately for each user.
Quotas are reset every day at midnight.

Reset the quota of a user or IP from GUI and CLI.

From GUI.

Go to Monitor -> Fortiguard Quota.
Select the user entry, and select 'Reset Quota'.

In 6.4 version, do the same under Users & Devices ->  FortiGuard Quota Widget.
Expand the widget to full screen to select source and reset quota.

From CLI.

Run the command '# dia webfilter quota list' to verify the user list.
# dia webfilter quota list

VDOM         User                                           Web Filter Profile                       Quota ID             Remaining Quota

root                                  default                                  1                    268378285 Bytes
Total number of quotas: 1.
This will list all the user information.
Run the below command to reset the entry.
# exe webfilter quota-reset<webfilter-profile> <endpoint (user name or IP)>

# exe webfilter quota-reset default