In v2.1+ you are able to modify weapons and subsystems to change their stats, or give them unique abilities. To do this you will need to find a engineer and earn his/her trust by completing a series of quests. Once you have earned their trust, you can use their workshop by landing on their settlement and using the Engineer's workshop option when landed;
There are a number of different Engineers, each offering their own specialisations. Some engineers prefer weapon modification, others sub-systems, with various grades. Grade 1 modifiers are typically fairly weak - offering usually minor changes, however grade 5 modifiers offer some huge advantages. When viewing the Engineers workshop you can either view relevant recipes or browse all (for future use);
Once you have selected a piece of equipment to modify, you will be given a list of possible modifications. Each modification has a primary effect - to increase range, capacity, damage, etc, with a grade (strength of modification - 1 being low, 5 highest). Weapon modifications can also have secondary effects that can further change the equipment - sometimes for worse, sometimes better.
In the above recipe, you can see that this modification can increase the maximum range anywhere from 2% to 39%, but other primary effects will be effected in a negative way. Each modification requires a set amount of resources to do, and can be rolled multi-able times to get the desired effects.