Tuning cap replacement _ help

How do I replace tuning knob cap w/out scratching it? I ordered a replacement knob (just 4 the cap).In trying 2 remove cap from tuning knob 2 add 2 my lx198's original knob? I could just swap the whole knob but I'd like 2 use mine.and do so w/out tearing up the part I just purchaced.any advice would help!

Aha struck gold...found the exact piece I need! My westie has been missing a lockdown washer (black & triangular). how I wish I could say I stumbled in a store n found it however, in all my great wisdom.the day my guitar arrived.i put on new strings n some1 walked up beside me & was setting a basket of clothes on my New guitar so, I picked it straight up n down, sending the locking washers flying across the room, behind a bed.so out with the whole bed & everything under it yet NO WASHER! Searched & searched adding 2 my frustration.Wondering if it was even on the spectrum lx when it arrived. 2 my great delight,as I was under that bed 2day it magically appeared...awesome!! Yet I made my own replacement, nothing beats the original.