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You can't really stop it from going to seed. It's a biennial, and goes to seed as a means of repopulating. Once the seeds have been shed, it will die. You MAY be able to prolong it by removing the seed stem. Harvest leaves for cooking from first-year plants only and allow at least one or two second-year plants to fulfill their flowering/seed producing cycles to replenish your garden.

Here is an article that you may find helpful: https://www.gardeningknowhow.com/edible/herbs/parsley/tips-on-how-to-grow-parsley.htm

Wait until they dry on the plant, and then harvest them to plant again.

Trim herbs as you use them or prune as needed. Pruning naturally stimulates plant growth. Pruning herbs can be as simple as pinching back growing tips to encourage branching; removing, or deadheading, spent flowers; or harvesting a major portion of plant growth for drying and preserving. Pinching back herbs throughout the growing season is the best way to maintain healthy, vigorous, attractive plants. Prune branches just above a leaf node or above another branch where you want to force new growth to occur. It is important to deadhead herbs to keep plants growing and to encourage more flowering.

You should put your plant in a pot one size larger than the one it is currently in. This article will also help: https://www.gardeningknowhow.com/edible/herbs/hgen/growing-herbs-in-containers.htm

You can dry it whenever you harvest it, which is all season long. This article will help you with drying it:
https://www.gardeningknowhow.com/edible/herbs/hgen/how-to-dry-herbs-various-methods.htm