Postponing a match can easily be done in The FA Matchday App, if you don't have access to the app it can also be done within FA Full-Time - guidance available here: Completing Match Returns in Full-Time
Please note: If the option to postpone is greyed out then this means your league have the setting which allows team admins to change fixture status switched off in Full Time. Please contact your league if you wish to have this changed.
Video guidance is available below and written guidance further down the page.
1. Login to The FA Matchday App
2. Go to the fixtures tab
3. Click the "Setup" tab next to the fixture you wish to postpone
4. Scroll down to the bottom of the page and click "Game Status"
5. Set the "Game Status" to 'postponed' and then select your postponement reason > then click 'done'
6. Remember to click "Confirm and tell squad" or else this won't be saved. Once you've clicked this button it will then inform your team that the match is postponed and update Full-Time