The Azure reservation is fixed to the SKU size that you choose at the time of taking it out and fixed for the duration of the term chosen so ideally you want to determine the correct size first.
https://learn.microsoft.com/en-us/azure/cost-management-billing/reservations/prepay-app-service
You do have the option to exchange your reservation for a new size however so long as it is of higher value, please note this will start a new 1 or 3 year commitment term. So in other words you can increase the SKU size and exchange the reservation of the new size also.
https://learn.microsoft.com/en-us/azure/cost-management-billing/reservations/exchange-and-refund-azure-reservations
Please note, if you want to scale up your App Service Plan then you can do so at any time, just note this would mean that your new App Service Plan will be charged at the normal pay-per-hour pricing model and you will also still be charged for the original Azure reservation commitment but for testing purposes this might be of interest.