On Debian the convert utility from ImageMagick is missing HEIC support (at least in the stable version I’m running now). Luckily there is an easy and automated way to install a newer version that has all the bells and whistles:
IMEI - ImageMagick Easy Install
Running it is as easy as:
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git clone https://github.com/SoftCreatR/imei.git
cd imei
./imei.sh --checkinstall
This will create .deb files which you can install with dpkg -i.