Assume I have a flat file connection manager called FFCM and I'd like to update the ConnectionString property to C:\ssisdata\output\B.txt
dtexec.exe
/file Package2.dtsx
/set
\Package.Connections[FFCM].Properties[ConnectionString];C:\ssisdata\input\B.txt
These are case sensitive values so the connection manager must be called FFCM. If there is a ffcm, this wouldn't work.
If you have to deal with spaces in the path, honestly, it's easier to rename the path than to deal with it but I believe the entire argument needs be in double quotes, not just the portion after the semicolon
dtexec.exe
/file Package2.dtsx
/set
\Package.Connections[FFCM].Properties[ConnectionString];C:\ssisdata\input\B.txt
/set
\Package.Connections[FFCM2].Properties[ConnectionString];C:\ssisdata\input\BC.txt
/set
\Package.Connections[MyDB].Properties[ServerName];localhost
How do I determine what the thing is after the /set
By default, F4 will bring up the Properties window for whatever you have clicked on in the SSIS editor. Look for the PackagePath property of a given object and that identifies the "base" path.

From there, you will need to access the Expressions collection to identify the property you'd like to set ConnectionString

Thus making the full path we will SET via command line as
\Package.Connections[FFCM].Properties[ConnectionString]