FWIW ...
Is the cable a traditional stainless steel cable OR is it a wire for an electronic shifter?
Regardless, you probably do NOT need to remove the "cover" to replace the rear derailleur cable ...
BTW. Obviously cables do not last forever AND I know that some people log thousands of miles per year, but why are you replacing your 2015 bike's cable?
Have you logged 10,000+ miles on your bike?
Are you changing shifters?
______?
YOU can either
fish the cable through by attaching a sufficiently long piece of ~
1mm nylon cord OR other sufficiently strong cord (
e.g., fishing line) or multi-stranded wire ... heck,
dental floss is probably strong enough ...
Attach the "cord" VERY SECURELY to the end of the cable ... three-or-more inches which are wrapped-and-well-taped to the derailleur cable should be good enough ...
PULL the old cable out of the frame until the "cord" is accessible at both ends ....
Securely attach the end of the new cable to the "cord" ...
Pull the cord back through the frame ...
Attach to the rear derailleur ...
Adjust tension ...
Done!
IF you have
already pulled the old derailleur cable out of the frame, then you should be able to feed your new cable through the Down Tube and guide it through to the chain stay AFTER you remove the crankset (
and BB's sleeve, if present) ....
if you have trouble guiding the cable into the chain stay then you can put a "fish" tape/cord through the chain stay where it exists near the rear derailleur, etc.