tailgate / trunk stuck fast

[Philaggio]

Hi. I have a Honda Civic Y Reg. The tailgate / trunk / boot - whatever you want to call it - has got stuck. The central locking by using the fob or inserting the key in either of the locks works fine otherwise. This occurred before about 3 years ago and a garage fixed it by "replacing a mechanism" (that is a quote from my partner). I assume it was door lock mechanism/actuator. Any ideas I how I can get this fixed at low cost? E.g. is it simple enough to order a part myself and replace? If applicable. can anyone provide a link to an instruction on how to fix it?