Here's how you can do this with a Workflow:
Go to Setup and then click on Workflow Rules.
Create a new Workflow Rule and select MemberPlan as the object.
Give the Workflow Rule a name, for example, "Update Account Insurance Lookup".
Add a new Criteria to start the Workflow when the "Primary" field equals "True".
Add a new action of type "Field Update" to update the Account object.
In the "Field to Update" section, select the "Insurance" lookup field.
In the "New Value" section, select the MemberPlan as the value.
Save and activate the Workflow.

OR a trigger
 

trigger MemberPlanTrigger on MemberPlan (after update) {
    Set<Id> accountIds = new Set<Id>();
    
    // Loop through all MemberPlan records that have been updated
    for (MemberPlan mp : Trigger.new) {
        // Check if the Primary field is set to "True"
        if (mp.Primary__c == true) {
            // Add the Account Id to the set
            accountIds.add(mp.Account__c);
        }
    }
    
    // Retrieve the Accounts that were associated with the updated MemberPlans
    List<Account> accounts = [SELECT Id, Insurance__c FROM Account WHERE Id IN :accountIds];
    
    // Loop through the Accounts and update the Insurance field with the MemberPlan record
    for (Account a : accounts) {
        for (MemberPlan mp : Trigger.new) {
            // Check if the Primary field is set to "True" and the Account Id matches the Account Id on the MemberPlan
            if (mp.Primary__c == true && mp.Account__c == a.Id) {
                // Update the Insurance field on the Account with the MemberPlan record
                a.Insurance__c = mp.Id;
            }
        }
    }
    
    // Update the Accounts
    update accounts;
}