Installing updates via Bluetooth on your Garmin Forerunner 735XT is really slow. Garmin Express is an alternative, but it doesn’t have a Linux version. Luckily, you can install updates via USB by uploading them to the Forerunner as a mass storage device.
Step 1: Get the update version number
Go to Garmin Connect to determine which version is the most recent stable for your device. Log in to your account, go to Device & Sync Status (top right), click Software update available. Note the Latest version.
Step 2: Get the update file
There are many update files on this site, although it looks very unofficial. You can also look up the name of the update there and then search for it to find it on Garmin’s servers. For example, that’s how I found the update to version 8.30 for the Forerunner 735XT.
Download the update file. If it’s zipped, unzip it to get the .gcd file.
Step 3: Connect your Garmin device via USB
Plug your Garmin device into your computer via USB. It will load as a mass storage device. Open the GARMIN folder. Upload the .gcd update file there. Rename the .gcd file to GUPDATE.GCD.
Safely unplug the device from the computer. This can take a while, because the Garmin device doesn’t have very fast disk I/O. Do not unplug it until your computer says it’s safe. If necessary, use the ‘sync’ command from the command line to force the cached writes to synchronize.
Step 4: Install the update
Once you remove the device from the computer, it will prompt you to install the update. Approve the update and wait for the device to finish installation and reboot. You’re done!