1. A printing apparatus configured to execute print processing on a sheet, the printing apparatus comprising: a connection interface configured to connect a post-processing unit which executes post-processing on a sheet on which the print processing has been executed, wherein the post-processing unit holds firmware; a memory configured to store control firmware for controlling operation of the printing apparatus and update firmware for updating the firmware held by the post-processing unit, wherein the memory stores the update firmware before the post-processing unit is connected to the connection interface; a presetting unit configured to preset, according to a user's instruction before the printing apparatus obtains version information regarding the firmware from the post-processing unit, wherein the presetting unit is configured to preset a presetting, as a preset update method, to select either one of selectable options including a first presetting regarding an update method in which update of the firmware held by the post-processing unit with the update firmware is not permitted, wherein the first presetting is preset before the printing apparatus obtains, from the post-processing unit, the version information of the firmware held in the post-processing unit, a second presetting regarding an update method in which, in a case where a version of the firmware held by the post-processing unit is only upgraded, the update of the firmware held by the post-processing unit with the update firmware is permitted, wherein the second presetting is preset before the printing apparatus obtains, from the post-processing unit, the version information of the firmware held in the post-processing unit, and a third presetting regarding an update method in which, in both cases where the version of the firmware held by the post-processing unit is upgraded and the version of the firmware held by the post-processing unit is downgraded, the update of the firmware held by the post-processing unit with the update firmware is permitted, wherein the third presetting is preset before the printing apparatus obtains, from the post-processing unit, the version information of the firmware held in the post-processing unit; an obtaining unit configured to obtain, from the post-processing unit, the version information of the firmware held in the post-processing unit after the presetting unit presets the preset update method; a first determining unit configured to determine whether the post-processing unit has been connected to the connection interface, when the printing apparatus is started; a second determining unit configured to determine whether the firmware held by the post-processing unit is to be updated, based on a result of presetting by the presetting unit and the version information of the firmware obtained by the obtaining unit, in a case where the first determining unit determines that the post-processing unit has been connected to the connection interface; and an updating unit configured to update the firmware held by the post-processing unit with the update firmware stored in the memory in a case where the second determining unit determines that the firmware held by the post-processing unit is to be updated, wherein the update is performed, without user's input regarding selecting a version of the firmware for the post-processing unit after the version information of the firmware held in the post-processing unit is obtained; wherein, in a case where the third presetting is preset by the presetting unit and a version of the update firmware is different from the version of the firmware held by the post-processing unit, the second determining unit determines that the firmware held by the post-processing unit is to be updated, in a case where the second presetting is preset by the presetting unit and the version of the update firmware is later than the version of the firmware held by the post-processing unit, the second determining unit determines that the firmware held by the post-processing unit is to be updated, in a case where the second presetting is preset by the presetting unit and the version of the update firmware is older than the version of the firmware held by the post-processing unit, the second determining unit determines that the firmware held by the post-processing unit is not to be updated, and in a case where the first presetting is preset by the presetting unit, the second determining unit determines that the firmware held by the post-processing unit is not to be updated, and wherein the presetting unit, the first determining unit, the second determining unit, and the updating unit are implemented at least in part by at least one processor.