Train by doing the stuff for each skill
It costs credits
Go straight down from the snyth home and go to the ground.
Train by doing, Do stuff that requires the stat you need. You get endurance for being hit in battle, and Dex for chasing the people that steal from you and stealing from others. Don't ask me about intelligence I only way I really know how to get intellect regularly is fixing the broken synth.
You can Technically change your appearance and parts whenever you want. But any single part can cost 1k and upward, go to the far right corner of the map and find the robot lover. You can also find cheaper parts but sometimes damaged parts in the market. I prefer the market cause you can save scum the part they give you, though it's locked to it's Rarity pool.
From the Synth home place go straight down and pay for a way down, there you can scavange and fight for stuff. DONT GO DOWN THERE TOO EARLY, YOU WILL DIE. Or at least I did, the pair of sex synths can tease you and I've yet to find (or I can't currently remember) a way to rid arousal mid battle. I recommend like 20 painkillers, the tease ability, and a handfull of endurance. Or just run around as to not gather suspicion, there are Atms down there so it's basically free credits.