set postgres password

charles@charles-composer:~$ su
Password:
root@charles-composer:/home/charles# su postgres
postgres@charles-composer:/home/charles$ \password postgres
No command 'password' found, did you mean:
 Command 'assword' from package 'assword' (universe)
password: command not found
postgres@charles-composer:/home/charles$ psql
psql (9.5.19)
Type "help" for help.

postgres=# \password postgres
Enter new password:
Enter it again:
postgres=# \q
postgres@charles-composer:/home/charles$