Upgrade kits can be used on clothing to add stats to them; clothing has different slot orientation among each item. You can salvage average+ quality kits for tools and precious ore.
You cannot equip multiples of an upgrade kit with the same effect on the same clothing item, however you can spread them across different items. When you have the same update kit on multiple clothing the effect is cumulative for passive effects, and will have multiple chances of firing (even at the same time) for chance based ones.
Kit Quality Rarity*
*Unofficial, maybe vary slightly from actual values.
Before you can equip a kit, you must first upgrade the clothing item you wish to equip it on with a Sewing workshop at least once to open a slot. Once the clothing is upgraded, the first slot will open, with the other 2 slots opening as you upgrade it further.
To equip a kit, open your profile and then drag the kit onto the slot you wish it to be on. You cannot remove a kit once equipped, but you can replace it.