i have 2 different apple id’s one on my iphone a good nd one in ipad. how can i just have one id to use on both devices

The short version is to sign out of the ID you don't want and then sign in with the one you do.

Sign out of iCloud on your iPhone, iPad, iPod touch, Apple TV, or Mac --> https://support.apple.com/HT208242

If another Apple ID is signed in you may need to enter the password for that ID to sign out of that ID first.

Click on this link to learn how to 'Sign in with your Apple ID' --> https://support.apple.com/HT204053

Set up iCloud on your iPhone, iPad, or iPod touch - https://support.apple.com/HT208681

For the long part...

Merging Apple Accounts or transferring data is not officially supported, nor can it be done easily or completely.

Apr. 2018 post by ASC helper Winston Churchill on transferring data from one Apple ID to another. https://discussions.apple.com/message/33333380#message33333380

Feb. 2021 tips from Eric Root https://discussions.apple.com/thread/252468781?answerId=254653802022#254653802022

Look into using Family Sharing to continue to be able to use items purchased with one Apple ID with other Apple IDs.

Family Sharing - https://support.apple.com/HT201060

Set up Family Sharing - https://support.apple.com/HT201088

You'll mostly lose access to purchases you made with the other ID unless you set up Family Sharing. You may have to change some settings, re-enter password, etc. It isn't in Apple's design that a person ever set up more than one Apple ID so they don't have a method for combining IDs. If anything, read this old version of their document:

* From an older version of the Apple ID FAQ - https://appleid.apple.com/faq

"I have multiple Apple IDs. Is there a way for me to merge them into a single Apple ID?

Apple IDs cannot be merged. You should use your preferred Apple ID from now on, but you can still access your purchased items such as music, movies, or software using your other Apple IDs."

I also have 2 different apple ids but one i use to download apps but I can’t remember that password n when I change my password it changes it for the other Apple ID. I need to figure out how to change my password for my other Apple ID. Can someone plz help me?