aromeropompa , you have a really nice bike with good components. I wouldn't change or "upgrade" anything just yet. The most effective (and economical) thing you can do to "refurbish" it would be to take it apart and
clean the components. Dirt, corrosion, rust etc. are the main culprits of a poor performing bicycle.
Start with the drive train.......before you buy a new chain and freewheel, you'll need the tools to remove those two components. Once off the bike, they should be soaked in mineral spirits overnight..........remove the derailleurs, cables and shifters, take them apart as far as you can and thoroughly clean them as well
https://www.parktool.com/product/min...category=Chain
https://www.parktool.com/category/cassette-freewheel