You should immediately change the cassette to 12-27 or anything with a lower gear up to 30T (if you can find one). That will make a significant difference without requiring that you change anything else.

Then consider a smaller chain ring up front - say a 36T instead of a 39T (again, if you can find one). That won't make as much difference, but it will help.

Then just keep riding. You'll surprise yourself when those mountains start to shrink.



(Consider your climbing techniques, too. E.g. Do you alternate standing and seated climbing?)