DIAGNOSIS You are attempting to install firmware intended for a different hardware version (e.g., R7000P) or a corrupted firmware file.
FIX Confirm you have the exact R7000 model. Check the label on the bottom. Download the correct firmware file directly from Netgear's support page for the 'Nighthawk AC1900 Smart WiFi Router (R7000)'. The file should be named like 'R7000-V1.0.xx.xx.img'. Do not use .chk files for standard updates. Log into the router's web interface. Go to Advanced > Administration > Firmware Update. Click 'Browse', select the .img file, then 'Upload'.
IF THAT DIDN'T WORK Your router may be running very old firmware that requires an intermediate update. Find the release notes for the firmware you want. If jumping multiple major versions, Netgear sometimes recommends installing an older version first. Download the immediate previous major version, install it, then install the latest.
HEADS UP Never interrupt a firmware update. The process takes about 3 minutes. The router will reboot twice. All lights will flash. Do not power off or use the internet during this time.