The brief:
Delay upgrading your items until you are doing Nightmares and Distortions regularly to earn the resources you need.
Until then, assemble two gear sets in your P-Equipment screen from your found items:
1) DPS set high in [Main Damage] (or [Critical Chance] and [Damage to Elite]) and
2) Support/Tank set high in [Main Defense] (or [Protection from Elite] and [Protection from Control])
Research builds carefully first: even excellent players disagree as the best combinations of attributes. Inspect other players' gear and read guides, then choose exactly what you want to make. You'll make new sets each invasion, so your decision is not permanent.
To change a weapon secondary stats, get a new one and get lucky. When a weapon is old, you can modify the generation and increase the main numbers, but that will only change the numbers, not what it does.
The deets
Upgrade current generation epic purple or yellow mythical gems and jewelry using Pure Matter, Marks, and tens of millions of credits. The items are in the fourth tab of your B-Bag. The upgrade interface is in your Bag tab for "Upgrade"
Step-by-step summary after you've decided what you need to make:
1) First, gather resources, including Sparks, [Credits], [Pure Matter], high-integrity epic gems or jewelries, and Marks from running many Nightmares, Distortions and other sources listed in the Invasion Calendar. Upgrading items is expensive: a perfectly rolled item might cost 5 million credits or more if you're unlucky.
2) Second, open the upgrade window from your Bag. Select and upgrade a high-integrity epic gem or jewelry item to mythical status using [Credits] and [Pure Matter], or if you are lucky enough to pick up a mythical item, use that. If the item already has some of your chosen attributes, even better. Current generation items offer better superiority buffs, but older gen items are cheaper to upgrade.
3) Third, use "Modification" to open a menu to change that item's qualities as you wish. Build guides elsewhere help you decide what you want. The qualities you lock with a click get a solid box and will not change when you click [Modify] and the others will. You have only a chance to get the quality(s) you want each time you modify ("roll") the item.
4) Fourth, fine tune the item using your Sparks of Simplicity (key stats), Morale (PvP buff), Frugality (saving credits, best when rolling only one or two qualities) and finally one Evolution (taking all qualities to maximum values).
5) Fifth, when you need high superiority for a prize or the small buff it gives, upgrade your CURRENT GENERATION mythical items' integrity using Marks. Each type of item uses an unchangeable type or types of Mark to upgrade. The interface shows you what you must spend.
6) Last, WEAR THE BEST, SELL THE REST. Sell Sparks of Luck, Memory, and Perfection. Sell excess Marks and Matter. Sell low rarity gems and jewelry. Sell older items you no longer need.
7) Repeat this process each invasion as the current generation rises. You get faster as you master the interface and become familiar with the sets you want to wear. Good luck