区切り文字で分割

$name = "aaabaaa"
$name = $name -split "b"

aaa aaa

区切り文字で分割（配列）

$name = "aaabaaa"
$name = $name.split("b")[0]

aaa

文字列の置き換え

$name = "aaabaaa"
$name = $name.Replace('b','a')

aaaaaaa