You will need to contact your Microsoft distributor to information on installing office application on Windows IoT. A list of distributors can be found here: https://github.com/MicrosoftDocs/windows-iot-public/blob/main/windows-iot-distributor-information/Windows_IoT_Distributor_Information%2010-13-21.pdf
Just a note from past experience with this topic: one can install a piece of Microsoft office for use with the Embedded/IoT device/application, but the device cannot be used as a general-purpose computer.