If you have your own .pth model file then just load it and finetune for the number of epochs you want.
import torch

model = get_model()
checkpoint = torch.load(path_to_your_pth_file)
model.load_state_dict(checkpoint['state_dict'])

finetune_epochs = 10 # number of epochs you want to finetune
for epoch in range(finetune_epochs):
         # assuming you have functions for training and validating models 
         train_model(model)   
         validate_model(model)