2.5.915 and below
The Signature algorithm used is SHA1. It is not possible to change the algorithm.
2.5.916 and above
The Signature algorithm used is SHA2 - SHA256
NOTE: It is down to the CA configuration which algorithm is used to sign the certificate. Even if the CSR signature is SHA1, the certificate the CA generates can still be signed by SHA256. All versions of 2.5.x support certificates signed with SHA256.