How to find, check and change file?

I have a doubt that I am new to python.
I have a file for example:

bla bla bla 
bla bla bla 5
bla bla bla 846:545
energia 3

I intend to read from the teste.txt file the energy information.
Of which I want to check the number that is, and the number 3, and if need be, change it to number 5.

You will have to read line by line and then rewrite the file:

arquivo = open("teste.txt","r")
linhas = arquivo.readlines()
arquivo.close()
linhas_a_escrever = ''
for linha in linhas:
    if "energia" in linha:
       lixo,valor_energia = linha.split(" ")
       if int(valor_energia) == 3:
           linhas_a_escrever += "energia 5n"
           continue
    linhas_a_escrever += linha
arquivo = open("teste.txt","w")
arquivo.write(linhas_a_escrever)
arquivo.close()

I imagine this is what you want to do.

A solution creating a new file with the changed value:

novo = open('novo_ficheiro.txt', 'w')

with open('ficheiro.txt', 'r') as ficheiro:
    for linha in ficheiro:
        nova_linha = linha
        if 'energia' in linha:
            nova_linha = 'energia %i' % 4
        novo.write(nova_linha)
novo.close()