how can i change encoding in template1 DB?

I installed postgresql by pacman (i'm using an ArchLinux Distr)
the default encoding of template1 is SQL_ASCII, i want to change this
one with UTF8 encoding

i try to use ALTER DATABASE but this statemanent hasn't the clause
encoding (while the CREATE DATABASE has it) so i can't modify the
template1 db by this one.

how can i do it?

many thanks

nando