a = ['2.40', '1970-1990', 'Austria'] #your old list (with the extra [] removed, they seem not to have a point... If you need them you can easily edit the code appropriately)
b = [] #a new list with the absolute values
for i in range(0, len(a)): #going through each of the values in a
try:
b += [abs(float(a[i]))] #trying to take the absolute value and put it in the new list (I have the float() because it appears that your 2.40 is a string. If you have it as an actual integer or float (such as 2.40 instead of '2.40') you can just use abs(a[i])
except:
b += [a[i]] #if taking the absolute value doesn't work it returns the value on its own.
print(b)