Hi Rainfo! Welcome to Goy. Am assuming you mean Pieris 'Forest Flame'. Here's the RHS advice:
1. Early flowering evergreen shrubs (Pruning group 8)

Timing: Prune immediately after flowering.
Examples: Berberis, box (Buxus), Camellia, Ceanothus, Choisya, Daphne, Hypericum, Mahonia, Pieris, Azalea (Rhododendron), Rhododendron, Viburnum tinus.
Pruning: Winter, spring and early summer-flowering evergreens such as Rhododendron and Camellia are best left unpruned except for removal of unsightly shoots and deadheading, unless some shaping is required. Do this by lightly cutting back shoots after flowering. Deadheading and removal of dead, damaged and diseased growth can be done at the same time.
Exceptions: Certain evergreen shrubs require a harder prune to increase flowering or to maintain their attractive shape. Examples include groundcover such as Mahonia aquifolium and Hypericum calycinum which look best sheared over in spring. Deciduous Viburnum - whether winter, spring or summer flowering - also fall within pruning group 8 and can be lightly pruned after flowering unless berries are required.

I agree with the pruning advice.
if its yellowing and in a pot it needs repotting in a bigger pot with fresh compost. if you cant repot then scrape the top 2" off and replace with fresh compost. give it a good feed and add some slow release fertilizer.

welcome to GoY too :o)