You can edit an individual product and adjust its lag time, then click Save and Export to Walmart.
If you want to update this value in bulk, you need to export your products to a file, either by using Bulk Action or via Tools > Import/Export Product. Then, modify a file to have only two columns, SKU and fulfillmentLagTime.
Adjust numbers for fulfillmentLagTime column, and upload a file. Then, you need to export your products to Walmart. Please keep in mind you may need to export them in small batches, large submissions (above 300 SKUs) usually fail.