PHP/Python Replace first string in a line with another string

PHP/Python Replace first string in a line with another string

I'm trying to replace the first ":" with "space" and I don't have a clue
on how to do it! Maybe you guys can help me.
In a text:
ab:cd:ef:gh:ij:kl:mn
12:ab:c2:f3 303:mx:
dk:ad:Y4:D3
12:31:44:aC:210 sda:
The output will be :
ab cd:ef:gh:ij:kl:mn
12 ab:c2:f3 303:mx:
dk ad:Y4:D3
12 31:44:aC:210 sda:
Maybe you could help me out.
Thanks in advance