How old is you acer/ how long has it been in its current spot?
Is it first time it is showing those signs?
What is your soil like?....do you know its PH?
Is that brick wall I can see in back ground?
Have had any other bigger plants nearby but are now cut back/removed so that your acer is more exposed situation than before?
Are those brown parts mainly on just one side of the plant (if so facing on...?)... or are they all over the place?
It is like detective work...millions of questions to find the problem...