To work on something that is already in good condition to improve it yet further. From Yorkshire, UK. Example: What are the new Pace suspension forks like? Oh, they took last year's one and fettled them. They've got grease ports, unobtanium stanchions, and a bit more travel.
FETTLE
Meaning of FETTLE in English
Slang English vocab. Английский сленговый словарь. 2012