How to modify blocktime of a running private ethereum network

I have a private Ethereum network that is running on geth 1.8 using PoA consensus. It consists of two nodes - one sealer node and one bootnode/RPC API node. When I created the genesis file I set the blocktime to 3s but it generates too much data this way and I want to set it to ~10s. How can I do this without loosing previous transactions and data?

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