1. You need to have "-console" in the command line options for Valheim.
2. Press "F5" ingame to open the console.
3. Type in the following commands from the console.
4. You can now press the following keys to toggle the following options:
Z - fly
B - no cost

There are also commands to spawn stuff in, but I advise you to read some of the excellent guides on developer commands available in the guides section for Valheim for those.