To change text in core Joomla or in an extension (other than installing a new language) it is advised to perform a language override in Joomla. To do this you need to go into the admin backend then in the menu under “Extensions” click on “Language Manager”
Let us assume that you want to change the First name (in billing address fields) to My Name.
Step 1: Go to Joomla admin->Extensions Manager->Language Manager->Overrides.
Before creating the override, set the Filter to ADMINISTRATOR (of the language that you wanted to create override. Say, your language is German. Then you should select the German (de-DE) Administrator in the filter)
Step 2: Click new to create a override
Make sure you have checked the For both locations
Make sure the location points to ADMINISTRATOR
Step 3: You can either search or directly enter the constant (if you knew already) to create the override.
Under the Search text you want to change, enter the following text (as you see in the site): Firstname and hit search.You will get the constant: J2STOREFIRSTNAME
Let us take J2Store as example. All the language constants of J2Store can be found in the following file/administrator/language/en-GB/en-GB.com_j2store.ini
Step 4: Now you have the constant. Enter the constant in the Language Constant box.
Step 5: Enter your value (in our example My Name) in the Text box.
Step 6: Save.
You can create as many overrides as you like in this method.