1. Enable developer mode from settings
2. Enter Studio and open the report you want to edit. Click on "More".

Note: If you don’t have Studio you can look for the report inside the "Reports" model. Use CNTRL + K + "/" to look for this model.

3. Change “Printed Report Name” field

Example: 'Sale number ' + (object.name) + ' for ' + (object.partner_id.name)

Here you can use python expressions

Explanation of the structure:

• Use quotes ('') in case you want to add specific text
• Use “+” to concatenate different elements
• Use object.field between parenthesis to specify a field to use. Make sure to use the technical name of the field

Or  

• Put all the text between quotes (''), 
• Inside this same text, specify the fields using “%s”, 
• Add “%” out of the quotes 
• Add all the technical names of the fields between parenthesis (adding “object”, and separated by commas as shown below)

Example: 'Sale number %s for %s' % (object.name, object.partner_id.name)

Note that you can retrieve fields from inside other related records like the name of the client in a sale order (object.partner_id.name), just make sure which fields are available in the corresponding model and use the technical name. You can even use new fields added with studio!

Tip: Enable developer mode from settings to see the technical name of the fields when positioning the mouse over the “?” symbol

Result of the example showed above: