To upgrade your Fang Spring, you need Fang Spring Shards.
With each upgrade, you need more Fang Spring Shards.
Below is a list of possible ways to gain these.
Gems
You can purchase Fang Spring Shards using gems.
1 Fang Spring Shard = 15 Gems
10 Fang Spring Shards = 125 Gems
100 Fang Spring Shards = 1,154 Gems
1000 Fang Spring Shards = 10,000 Gems
Chest
You can purchase a chest using guild coins to have a chance at
gaining extra Fang Spring Shards.
To purchase, click bag, click chests, click the third icon on right hand side.
The chest is called Magical Shard Box and costs 22,500 Guild Coins.
It randomly gives you 10 shards for any of the ruins or statues.