Simply break the loop when you find a[i-1] <= mins
.
ar = [2, 4, 6, 8 ,1]
mins = ar[-1]
for i in range(len(ar)-1, 0, -1):
if ar[i-1] > mins:
ar[i] = ar[i-1]
else:
ar[i] = mins
break
print(ar)
if mins < ar[0]: ar[0] = mins
print(ar)