Can I Change Print Order Size?

i did a search on the site, but no results came up to help answer my question. my apologies if this has been answered before.

my customer ordered a 5 x 7 of a print. she is a sweet friend, on a tight budget, and i would like to change her order to an 8 x 10 for her (this is the size she really wants, but cannot afford).

is there a way for me to change this on my end so that she is surprised when she receives her order? i don't mind paying the difference as it would come out of my profit.

i just want to change the 5 x 7 to an 8 x 10 ... nothing else.

thanks!:barb