Wasteland Remastered

Skills can only be raised by one of two ways: visiting a library to invest earned skill points, or using a skill repeatedly. I would suggest just going for this naturally through combat skills such as Brawling. A quick way to earn this later in the game though is to have someone with the Swim skill learned (Angela Death starts with it) jump into the river at the top of the map near Sleeper Base and be swept all the way to the lake in the south. You'll pop out next to Highpool and can go heal up the characters who took drowning damage using the Camp feature, then repeat this until the skill maxes out at level 6.

It's important to note skills can't progress unless you're a specific rank, so you'll need to have leveled up a few times or you'll still never progress a skill, which is why it's better to do this late in the game or try for it naturally.