To manage exchange gain or loss you have to have two accounts and exchange  journal and Odoo created theme by default if you used Generic chart of account and you can create theme manually as well as below:

- Foreign Exchange Loss: Its an expense account and you can create it from chart of account with the below values:

Code: Use the code start with 6 and you can see the last number start with six for example (641000)

Name: Foreign Exchange Loss

Type: Expenses

2-  Foreign Exchange Gain: This is an income account and you can create it from chart of account with the below values:

Code: Use the code start with 4 and you can see the last number start with four for example (441000)

Name:  Foreign Exchange Gain

Type: Income

After creating the account, create a new journal from Accounting App-> Configurations -> Journals

Name: Exchange Difference

Type:  Miscellaneous

Short Code: EXCH

Once you created the journal go to Accounting -> Configurations -> Settings and under the default accounts section, you will find " Post Exchange difference entries in: " under it set the below:

Journal: select the above created journal

Gain Account: Select the above created Gain account.

Loss Account:  Select the above created Loss account.

Then save the configuration. This all what you have to do!