Download the AppImage file "FreeCAD_0.21.0-Linux-x86_64.AppImage" from following website. (If your machine has an ARM CPU, select "FreeCAD_0.21.0-Linux-aarch64.AppImage".)
Release FreeCAD 0.21 · FreeCAD/FreeCAD
After downloading the file, move the AppImage file to a directory which you want to install FreeCAD at.
Then right-click the AppImage and select [Properties] to open Properties dialog. And check "Allow executing files as program" in "Permissions" tab.
After Permissions settings, double click the AppImage file to start FreeCAD.
On Ubuntu, FreeCAD can be install witn PPA. Add PPA of stable version FreeCAD and install FreeCAD with following commands on terminal.
After installing, you can start FreeCAD with command "freecad" on terminal.
In addition to above normal version repository, PPA (Personal Package Archives) of daily build version repository are offered. These developing repogitories were not stable but has newest features you can use. You can install it with following commands.
After installing, you can start developing version FreeCAD with command "freecad-daily" on terminal.