First off: Kill cows to obtain leather.
Second: Craft armor using your crafting bench.
Third: (Leather: helmet [5 Leather], chestplate [8 Leather], leggings [7 Leather], boots [4 leather]) can be crafted in 1, 2, 3 and 4 tiers crafting bench.
But if you want to upgrade your leather armor to iron or gold or diamond or any ore type:
• (Iron: helmet [5 Iron and 1 leather helmet], chestplate [8 Iron and 1 leather], leggings [7 Iron and 1 leather], boots [4 Iron and 1 leather]) can be crafted in 2, 3 and 4 tiers crafting bench.
• (Gold: helmet [5 Gold and 1 leather helmet], chestplate [8 Gold and 1 leather], leggings [7 Gold and 1 leather], boots [4 Gold and 1 leather]) can be crafted in 3 and 4 tiers crafting bench.
• (Diamond: helmet [5 Diamond and 1 leather helmet], chestplate [8 Diamond and 1 leather], leggings [7 Diamond and 1 leather], boots [4 Diamond and 1 leather]) can be crafted in a 4th tier crafting bench.
Above is an example, you can change the ore order or change types of ores. Leather part is apart if what should be added before getting your next armor upgrade.