Answer:
A horizontal translation of 5 units to the left. 
Step-by-step explanation:
Given the parent linear function: 

To shift vertically n units, we can simply add n to our function. Hence: 

So, a vertical shift of 5 units up implies that n=5. So: 

As given. 
However, to shift a linear function horizontally, we substitute our x for (x-n), where n is the horizontal shift. So: 

Where n is the horizontal shift. 
For example, if we shift our parent linear function 1 unit to the right, this means that n=1. Therefore, our new function will be: 

Or: 

We notice that this is also a vertical shift of 1 unit downwards. 
Therefore, we want a number n such that -n=5. 
So, n=-5. 
Therefore, it we shift our function 5 units to the left, then n=-5. 
Then, our function will be: 

Hence, we can achieve f(x)=x+5 from f(x)=x using a horizontal translation by translating our function 5 units to the left.