That depends on whether you concluded a contract with a minimum term of 24 months or just 1 month.
What is the exact name of the tarif and where / how did you conclude the contract?
@bs0 on the contract it says 24 months and tarif name is L boost online
Ordered online or in a shop?
If online you could cancel the contract within 14 days and conclude a new one, but you would lose the phone number. If in a shop, this is not possible.
You have a 24-month contract so you can't just change it after one month.
Hello @Bayrbilegt,
welcome to our o2 community
Could bs0 already answer your questions regarding possibilities to cancel your contract?
If you want to continue or ordered in a shop or more than 14 days ago online or via our hotline, then the earliest possibility to be able to change the tariff is after 5 months depending on available offers with an extension. A change of tariff without extension is also possible anytime, but often lower tariffs are not offered.
Best Regards,
Gerrit