console commands is the easiest way
In the steam valheim options, type -console in the launch options.
Ingame, press F5 to open the console commands

Easiest is probably
type devcommands
then debugmode
exit the console commands
Press B to disable any build costs and make yourself a free new helm. You could spawn in one, but this is a lot easier.
To disable, just press B again, and type in debugmode and devcommands again in the console commands.

Originally posted by Joe Montgomery:

I'd disable steam cloud, the larger saves get the more unreliable steam is at syncing both saves.

And yes, don't rely on steam cloud, it can severely mess up your saves.