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##JavaScript ES6, 63 58 52 49 bytes

c=>c.replace(/\w/g,x=>(15-`0x${x}`).toString(16))

Thanks to nderscore for saving 11 bytes!

JavaScript ES6, 63 58 52 49 bytes

c=>c.replace(/\w/g,x=>(15-`0x${x}`).toString(16))

Thanks to nderscore for saving 11 bytes!

##JavaScript ES6, 63 58 52 49 bytes

c=>c.replace(/\w/g,x=>(15-`0x${x}`).toString(16))

Thanks to nderscore for saving 11 bytes!

##JavaScript ES6, 63 58 52 49 bytes

c=>c.replace(/\w/g,x=>(15-`0x${x}`).toString(16))

Thanks to nderscore for saving 11 bytes!

##JavaScript ES6, 63 58 52 bytes

c=>c.replace(/\w/g,x=>"fedcba9876543210"[+('0x'+x)])

Based on my Japt answer (strange that not vice-versa :D)

Thanks to nderscore for saving 11 bytes!

##JavaScript ES6, 63 58 52 49 bytes

c=>c.replace(/\w/g,x=>(15-`0x${x}`).toString(16))

Thanks to nderscore for saving 11 bytes!