The Kings Road branch will close from 12.30pm on Monday, June 30, for the revamp, reopening on Monday, July 14, at 1pm.
The new-look post office will have open-plan counters. It will also welcome new operator Anita Sharma, who will replace retiring subpostmaster Paul Wahlia.
Saturday trading hours will be extended once it reopens, from 9am-12.30pm now, to 9am-5.30pm.
Post Office area manager Mark Jacobs said: “We understand how important Post Office services are to the people of Sunninghill and we are confident this new, modern Post Office service will meet the needs of the local community and secure services for the future.
“This modernisation is part of a major investment programme, the largest in the history of the Post Office and marks a commitment to no more branch closure programmes.”