When you authorise a connection to Bitbucket for retrieving repository information to add to your DeployHQ project, an authorisation is created within your Bitbucket account.
If you wish to connect to another Bitbucket account, you can head to Bitbucket, then login using your currently linked account details.
After that, head across to your main Bitbucket Settings page via your profile icon at the bottom-left of the screen, then over to OAuth within Access Management.
You'll find DeployHQ within the OAuth integrated applications section - just click the Revoke link on the right hand side.
Once you've revoked access, you'll be prompted to login when you next create a DeployHQ project, and you can then connect using your other Bitbucket account.