There are many easy ways to customize the NPCs for your home campaign.
Racial Traits. You can add racial traits to an NPC. For example, a halfling druid might have a speed of 25 feet and the Lucky trait. Adding racial traits to an NPC doesn't alter its challenge rating. For more on racial traits, see the Player's Handbook.
Spell Swaps. One way to customize an NPC spellcaster is to replace one or more of its spells. You can substitute any spell on the NPC's spell list with a different spell of the same level from the same spell list. Swapping spells in this manner doesn't alter an NPC's challenge rating.
Armor and Weapon Swaps. You can upgrade or downgrade an NPC's armor, or add or switch weapons. Adjustments to Armor Class and damage can change an NPC's challenge rating.
Magic Items. The more powerful an NPC, the more likely it has one or more magic items in its possession. An archmage, for example, might have a magic staff or wand, as well as one or more potions and scrolls. Giving an NPC a potent damage-dealing magic item could alter its challenge rating.