operate only on filtered elements in an array in python

Consider this simple problem: in a list of integers I need multiply all even number by 10. I can certainly do element-wise operation such as:

[if x%2==0: x=x*10 for x in arr]

But what if I the operation has to be operated on the array level? The trouble I am having is after the operation on the filtered array, how do I nicely put them back to the original array?

Code example:

arr=np.arange(1,10) # the original array array([1, 2, 3, 4, 5, 6, 7, 8, 9])
filter1 = arr%2==0  # the filter 
arr1=arr[filter1]   # the filtered array array([2, 4, 6, 8])
arr1=arr1*10        # the 'array'-wise operation array([20, 40, 60, 80])


# this is the part I am trying to improve
i=0
j=0
arr2=[]
for f in filter1:
    if f:
        arr2.append(arr1[i])
        i=i+1
    else:
        arr2.append(arr[j])
    j=j+1

# output arr2: [1, 20, 3, 40, 5, 60, 7, 80, 9]