How i maintain my battery health to 100%

It is not possible to maintain 100% since all batteries decline over time. In the case of iPhone batteries, you should expect the maximum capacity to decline at an average rate of about 1% a month.

So, if the phone is 12 months old, the maximum capacity would on average be about 88%.

Apple specifies that once the battery reached 80%, it should be replaced.

Take your phone to an Apple Store to have a new genuine Apple battery installed.

You can’t

Rechargeable batteries have limited number of charge cycles.

The battery health is going to go down over time no matter what you do.

It appears right now that your Battery Health is 77%, which is below the threshold Apple sets for replacement. They recommend replacement at 80%. You need a new battery right now.