Could you confirm what is the current version of this AP and what version you are upgrading to.
Error indicate issue in firmware file (chksum error means file may be got corrupted during the download or while moving from one location to another).
Try to download firmware image again, check and make sure MD5 checksome is same as mentioned in the support site firmware download link page.
Avoid remote upgrade, try using CLI with TFTP.
fw set proto tftp
fw set host 192.168.0.10 <<< TFTP server IP
fw set control 126.96.36.199.1363.bl7 <<< Firmware file name