As it says on the tin - have an advance ticket for tomorrow - need to change the out portion. Its from Nailsea to Reading
Nailsea does not have a ticket office
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It does, and it is open from 0630 to 0930 (confirmed on FGW▸ website), so if your ticket was booked on the 0644 then in theory you have enough time to present it to Jacob and get it changed before departure time. Though if you want to leave on the earlier train then you are stuck as it departs before 0630.
Yatton is open until 1230 on weekdays.