Instead of opening the text file in "r+" mode, would opening it in "a+" mode and writing to the file be an option?
So that would look like this:
#Editing the if condition only
if lol2 == "yes":
#Indentation is off for some reason, this should be indented below the if statement
fo = open(lol.lower() + ".txt", "r+")
string = fo.read()
fo.close()
# Finding where the colon is located
index = string.find(":")
# Slicing where the :new:old is
aStr = string[index:]
main = raw_input("Type in new stock price: ")
# Putting the string slices together
main = string[:index]+":"+userinput+aStr
fo = open("test.txt","w+")
fo.write(main) #This would add the raw input at the end of the file.
fo.close()
themain()