I tried finding a similar thread on the forum, but I couldn't find anything so I'm sorry premptively if there is a forum post about my issue that I missed. 

My character is a Warlock and I'm trying to "level down" a spell I have learned in my list of spells.  On the DnD Beyond application for my character, it gives me the option of "leveling down" the spell in the top left corner when selected. From my understanding when using the application previously, a spell in a lower spell slot than my highest filled level slot doesn't require the use of a pact in order to use the spell.  If this mechanic isn't a true mechanic as a warlock class then just having that understanding would be helpful.  However, if this mechanic is correct and I can do this, I can't lower the spells through the application itself and I'm wondering if this is a glitch in the app or if I need to do something different when "Learning a spell".  When I try to unlearn a spell and relearn it, it automatically puts it in my highest spell level slot regardless of what level the spell level comes from (with the exception of cantrips).

Any time you cast a spell, it uses a spell slot. There’s no way around it. 
Warlocks automatically cast their spells at higher level, as they gain levels in the class. But there is no official mechanic to reduce spell power and cast more of them. 
You get 2 spells/short rest for most of your career (though it does go up to 3 and then 4 if your campaign goes on long enough), and that’s it.

This is helpful thank you.  I must have misunderstood what I observed in the application earlier.

Are you talking about a multiclassed character who has both regular spell slots, and also Warlock pact slots?

If you're playing a pure Warlock then your pact slots have a fixed level that increase as you gain warlock levels. So if you're a 3rd-level Warlock for example, then your pact slots will be 2nd-level, so you may only see a cantrips section, and a 2nd-level spells section in your list, because the minimum level at which you can cast your spells is 2nd-level so there's no need to see them listed at 1st-level as well.

However, there are a few things that can change this:

• Eldritch Invocations: Some spells gained via invocations do not use your pact slots, so the will always cast at their lowest level. This means 1st-level spells will appear in the 1st-level spells section, and so-on, even if your pact slots are higher level than that.
• Magic items and feats that grant spells may grant them at different levels to your pact slots.
• If you multiclass into another casting class, such as Bard or Sorcerer, then you'll gain "ordinary" (non-pact magic) spell slots so will gain 1st-level slots, then 2nd etc. as normal. In this case your warlock spells should start appearing at the lowest level at which they can be cast, as you can cast them using any spell slots you have (not just pact slots). For example, say you have three levels in Warlock (2nd-level pact slots) and one level in Bard, and you have armor of agathys on your spell list. Since it's a 1st-level spell, it'll appear as such as you can cast it using the Bard's 1st-level spell slots if you want to, or you can upcast it using your pact slots as normal if you prefer.

Hope that clears it up more than it confuses things!