Use A/B test feature in the 'Add details' section of the campaigns to find the subject line which works for you.

Here's how its done: 

Step 1: Start creating the new campaign and select the template.  

Step 2: Click Next, and move to the Add details section. 

Step 3: In this section, 

- Add Campaign Name, 

- Add Subject line 

- Click on the '+'icon on the right to add another subject line for the campaign. 

This way you can add as many subject lines you wish to test for this campaign. 

- Add Pre-header to inform more about the email. 

- Add Sender Name to let your contacts know who is sending the email.

- Add email address in Reply to, to get your contact's replies on the email. 

Step 4: Select the relevant sender Id and Click Next to complete the campaign flow by selecting the list and finally setting it live.