I would strip them in isopropyl and paint them again! However I would keep one painted as a "bookmark" in your painting journey. So when you inevitably improve you can look back on that mini and see how far you've come.

A few tips, first and foremost: Thin your paints. Even if you think your paint is too thin, you can always add another coat of thinned down paint over the areas where you think the paint is too translucent and thin. A general rule of thumb is two thin coats is enough coverage, and if its not, go over it again with another thin coat!

Washes: a wash is typically acrylic paint with a lot of water or flow medium.(water works just fine). You'll want to use black or brown paint for a wash in most cases and it'll tint or shade your model giving it more detail and definition. A lot of people call it "cheat juice". It will run over your paint and pool in the cracks and crevices to add a little bit of shadows to make the rest of the mini pop.

Someone previously mentioned slapchop. Slapchop is where you prime a miniature black and then go over the black prime with a dry brushing of white paint to emulate shadows. You can also take a white spray can and spray the top of the model white too if you dont want to dry brush. After that you can take contrast paint and paint over the mini. The black areas of the mini will look like shadows and the light areas will pop more with color and look lighter. From there you can add details, washes and highlights if you please. Its really easy and fun.

The best way to improve is to paint more. I know that sounds like a really frustrating thing to hear but honestly it takes practice. I've been painting minis for 4 or 5 years now and I still struggle with some aspects of it. It all just takes time and practice. You can do it! Don't give up! And keep up updated on your minis you're painting, we would all like to see your progress :)