how to config the ecdhe signature algorithm?

[shi36hao@163.com]

i use the wireshark to track the openvpn connection packet, i found that the ec diffie-hellman params is like that:

TLSv1.2 Record Layer: Handshake Protocol: Server Key Exchange
Content Type: Handshake (22)
Version: TLS 1.2 (0x0303)
Length: 149
Handshake Protocol: Server Key Exchange
Handshake Type: Server Key Exchange (12)
Length: 145
EC Diffie-Hellman Server Params
Curve Type: named_curve (0x03)
Named Curve: secp256r1 (0x0017)
Pubkey Length: 65
Pubkey: 046b11e8940554dc460781b8a4381ab5c59007531b6ad3ad...
Signature Hash Algorithm: 0x0603
Signature Hash Algorithm Hash: SHA512 (6)
Signature Hash Algorithm Signature: ECDSA (3)
Signature Length: 72
Signature: 3046022100eff27499dd771953af98597048183cd53b6f8c...

the ec diffie-hellman signature hash algorithm is sha512, how should i config it to sha256?
Is there any one can help me， thanks。