Hi @anaspirant
If the charging port (aka [https://www.google.com/search?client=firefox-b-d&q=HP+EliteBook+755+G3+dc+in+jack|DC jack] - supplier¹ examples only) is only loose you can resolder it on to the systemboard.
If the charging jack is faulty e.g. fractured inner +ve connector perhaps, then it will have to be replaced as it cannot be repaired.
In either case to do the job properly the systemboard will have to be removed from the laptop so the repair can be done.
Here's the [http://h10032.www1.hp.com/ctg/Manual/c04853478.pdf|maintenance and service guide] for the laptop.
Go to p.71 to view the procedure to remove the systemboard from the laptop.
¹ Search online for ''Elitebook 755 G3 DC-In jack'' to find suppliers that suit you best.