Can't upgrade e400 to 3.5.2. Not enough space?


I tried to update via cnmaestro a couple of e400, it failed with 

SW update file download failed

If I tried to upload/upgrade from AP I got:
Upgrade Status : 413 - Request Entity Too Large</title> </head> <body> <h1>413 - Request Entity Too Large

With a third AP it upgraded fine, without problems.

How can we free some space from AP's?

Thank you

"Solved" rebooting the device.
Available Memory: before reboot: 37%, after reboot 59%.

What's going on here?
I can't reboot my devices twice just to upgrade them.

I requested tech-support to analyse and understand the cause.

Thank You for the udpate.