that depends:
L series:
The relay is located under the engine compartment fusebox, behind the engine management ecm, on the left hand side of the engine compartment.
disconnect battery -ve, unscrew 2 retaining bolts and position the engine compartment fusebox to one side, release the relay holder from the battery carrier and pull from the relay out of the wiring connector.
TD4:
The glow plug ECM relay is located in the left hand corner of the engine compartment, disconnect battery -ve, slacken 5 retaining screws and remove the lid from electrical box, disconnect wiring from ECM/relay unit, where fitted, unscrew the retaining nut and disconnect the lead to the ECM. remove the ECM from the box
refitting is the reverse of removal
(thank you to haynes, page 5C.3, section 4, one spanner rating.)
good luck