Normally, when upgrading the image of an AP, you can use the pre-image download feature to reduce the amount of time the AP is unavailable to serve clients. However, it also increases the downtime because the access point cannot serve clients during an upgrade. The Pre-image download feature can be used to reduce this downtime. However, in the case of a branch office set up, the upgrade images are still downloaded to each access point over the WAN link, which has a higher latency.
A more efficient way is to use the FlexConnect AP Upgrade feature. When this feature is enabled, one access point of each model in the local network first downloads the upgrade image over the WAN link. It works similarly to the master-slave or client-server model. This access point then becomes the master for the remaining access point of the similar model. The remaining access points then download the upgrade image from the master access point using the pre-image download feature over the local network, which reduces the WAN latency.